Semiconductor device and method for manufacturing the same

ABSTRACT

An object is to provide a semiconductor device including an oxide semiconductor with stable electric characteristics can be provided. An insulating layer having many defects typified by dangling bonds is formed over an oxide semiconductor layer with an oxygen-excess mixed region or an oxygen-excess oxide insulating layer interposed therebetween, whereby impurities in the oxide semiconductor layer, such as hydrogen or moisture (a hydrogen atom or a compound including a hydrogen atom such as H 2 O), are moved through the oxygen-excess mixed region or oxygen-excess oxide insulating layer and diffused into the insulating layer. Thus, the impurity concentration of the oxide semiconductor layer is reduced.

TECHNICAL FIELD

The present invention relates to a semiconductor device using an oxidesemiconductor and a manufacturing method thereof.

Note that the semiconductor device in this specification refers to allthe devices which can operate by using semiconductor characteristics,and an electro-optical device, a semiconductor circuit, and anelectronic device are all semiconductor devices.

BACKGROUND ART

A technique of forming a thin film transistor (TFT) by using a thinsemiconductor film that is formed over a substrate having an insulatingsurface has attracted attention. A thin film transistor is used for adisplay device typified by a liquid crystal television. Besides asilicon-based semiconductor material which is known as a material for asemiconductor thin film applicable to a thin film transistor, an oxidesemiconductor has attracted attention.

As a material for the oxide semiconductor, zinc oxide and a materialcontaining zinc oxide as its component are known. In addition, a thinfilm transistor including an amorphous oxide (oxide semiconductor)electron carrier concentration of which is lower than 10¹⁸/cm³ isdisclosed (see Patent Documents 1 to 3).

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2006-165527-   [Patent Document 1] Japanese Published Patent Application No.    2006-165528-   [Patent Document 1] Japanese Published Patent Application No.    2006-165529

DISCLOSURE OF INVENTION

However, a deviation from the stoichiometric composition in an oxidesemiconductor arises in a thin film formation process. For example,electrical conductivity of an oxide semiconductor is changed due toexcess and deficiency of oxygen. Further, hydrogen or moisture thatenters the oxide semiconductor thin film during the formation of thethin film forms an oxygen-hydrogen (O—H) bond and functions as anelectron donor, which is a factor of change in electrical conductivity.Furthermore, since an O—H is a polar molecule, it causes variation incharacteristics of an active device such as a thin film transistorformed using an oxide semiconductor.

In view of such problems, it is an object of an embodiment of thepresent invention to provide a semiconductor device including an oxidesemiconductor with stable electric characteristics.

In order to prevent variation in electric characteristics of the thinfilm transistor including an oxide semiconductor layer, impurities suchas hydrogen, moisture, hydroxyl, or hydride (also referred to as ahydrogen compound) which cause the variation are removed from the oxidesemiconductor layer.

An insulating layer having many defects typified by dangling bonds isformed over an oxide semiconductor layer with an oxygen-excess mixedregion or an oxygen-excess oxide insulating layer interposedtherebetween, whereby impurities in the oxide semiconductor layer, suchas hydrogen or moisture (a hydrogen atom or a compound including ahydrogen atom such as H₂O), are moved through the oxygen-excess mixedregion or the oxygen-excess oxide insulating layer and diffused into theinsulating layer having defects. Thus, the impurity concentration of theoxide semiconductor layer is reduced.

Since an insulating layer having many defects has a high binding energyto hydrogen or moisture (a hydrogen atom or a compound including ahydrogen atom such as H₂O) and these impurities are stabilized in theinsulating layer having many defects, these impurities can be diffusedfrom the oxide semiconductor layer into the insulating layer havingdefects, whereby these impurities can be removed from the oxidesemiconductor layer.

Further, the mixed region or the oxide insulating layer, which isprovided between the oxide semiconductor layer and the insulating layerhaving defects, includes excess oxygen, and thus have many oxygendangling bonds as defects and has high binding energy to impurities suchas hydrogen or moisture (a hydrogen atom or a compound including ahydrogen atom such as H₂O). Therefore, when the impurities are diffusedfrom the oxide semiconductor layer into the insulating layer havingdefects, the oxygen-excess mixed region or the oxygen-excess oxideinsulating layer functions to facilitate the diffusion. On the otherhand, when the impurities which have been removed from the oxidesemiconductor layer and diffused into the insulating layer havingdefects move back toward the oxide semiconductor layer, theoxygen-excess mixed region or the oxygen-excess oxide insulating layerfunctions as a protective layer (a barrier layer) which is bound to andstabilizes the impurities so as to prevent the impurities from enteringthe oxide semiconductor layer.

Thus, impurities such as hydrogen or moisture (a hydrogen atom or acompound including a hydrogen atom such as H₂O) in the oxidesemiconductor layer are diffused into the oxygen-excess mixed region orthe oxygen-excess oxide insulating layer.

Thus, the oxygen-excess mixed region or the oxygen-excess oxideinsulating layer removes impurities, such as hydrogen, moisture,hydroxyl, or hydride (also referred to as a hydrogen compound) whichcause variation, from the oxide semiconductor layer, and furtherfunctions as a barrier layer which prevents the impurities which havebeen diffused into the insulating layer having defects from entering theoxide semiconductor layer again. Consequently, the impurityconcentration of the oxide semiconductor layer can be kept low.

From the above, a thin film transistor including an oxide semiconductorlayer in which impurities such as hydrogen, moisture, hydroxyl, orhydride (also referred to as a hydrogen compound) which cause variationare reduced has stable electric characteristics, and a semiconductordevice including the thin film transistor can realize high reliability.

The mixed region is a mixed region of materials included in the oxidesemiconductor layer and in the overlying insulating layer havingdefects. By providing the mixed region, an interface between the oxidesemiconductor layer and the insulating layer having defects is notclearly defined; thus, diffusion of hydrogen from the oxidesemiconductor layer into the insulating layer having defects isfacilitated. For example, when a silicon oxide layer is used as theinsulating layer having defects, the mixed region includes oxygen,silicon, and at least one of the metal elements included in the oxidesemiconductor layer. As the oxygen-excess oxide insulating layer, asilicon oxide layer (SiO_(2+x), where x is preferably equal to orgreater than 0 and less than 3) can be used. The thickness of the mixedregion or the oxide insulating layer may be 0.1 nm to 30 nm (preferably,2 nm to 10 nm).

The oxide semiconductor layer, the oxygen-excess mixed region, theoxygen-excess oxide insulating layer, and the insulating layer havingdefects are preferably formed in a film-formation chamber (a processchamber) in which the impurity concentration is lowered by evacuationwith a capture-type vacuum pump such as a cryopump. As a capture-typevacuum pump, for example, a cryopump, an ion pump, or a titaniumsublimation pump is preferably used. The capture-type vacuum pumpfunctions to reduce the amount of hydrogen, water, hydroxyl, or hydridein the oxide semiconductor layer, the oxygen-excess mixed region, theoxygen-excess oxide insulating layer, and the insulating layer havingdefects.

Each of sputtering gases used in the formation of the oxidesemiconductor layer, the oxygen-excess mixed region, the oxygen-excessoxide insulating layer, and the insulating layer having defects ispreferably a high-purity gas in which impurities such as hydrogen,water, hydroxyl, or hydride are reduced to such a degree that theconcentration thereof can be expressed by the unit ppm or ppb.

In the thin film transistor disclosed in this specification, a channelformation region is formed in the oxide semiconductor layer, in whichthe hydrogen is set equal to or less than 5×10¹⁹/cm³, preferably equalto or less than 5×10¹⁸/cm³, and more preferably equal to or less than5×10¹⁷/cm³; hydrogen or O—H group is removed; and the carrierconcentration is equal to or less than 5×10¹⁴/cm³, preferably equal toor less than 5×10¹²/cm³.

The energy gap of the oxide semiconductor is set to be equal to orgreater than 2 eV, preferably equal to or greater than 2.5 eV, morepreferably equal to or greater than 3 eV to reduce as much impurities,such as hydrogen which form donors, as possible, and the carrierconcentration of the oxide semiconductor is set to equal to or less than1×10¹⁴/cm³, preferably equal to or less than 1×10¹²/cm³.

When the thus purified oxide semiconductor is used for a channelformation region of a thin film transistor, even in the case where thechannel width is 10 mm, the drain current of equal to or less than1×10⁻¹³ A is obtained at drain voltages of 1 V and 10 V and gate voltagein the range of −5 V to −20 V.

An embodiment of the present invention disclosed in this specificationis a semiconductor device including: a gate electrode layer over asubstrate; a gate insulating layer over the gate electrode layer; anoxide semiconductor layer over the gate insulating layer; a sourceelectrode layer and a drain electrode layer over the oxide semiconductorlayer; and an insulating layer having defects which is over the oxidesemiconductor layer, the source electrode layer, and the drain electrodelayer, and which is in contact with part of the oxide semiconductorlayer; in which an oxygen-excess oxide insulating layer is providedbetween the oxide semiconductor layer and the insulating layer havingdefects.

Another embodiment of the present invention disclosed in thisspecification is a semiconductor device including: a gate electrodelayer over a substrate; a gate insulating layer over the gate electrodelayer; an oxide semiconductor layer over the gate insulating layer; asource electrode layer and a drain electrode layer over the oxidesemiconductor layer; and an insulating layer having defects which isover the source electrode layer, and the drain electrode layer, andwhich is in contact with part of the oxide semiconductor layer; in whichan oxygen-excess mixed region is provided at an interface between theoxide semiconductor layer and the insulating layer having defects; inwhich the insulating layer having defects includes silicon; and in whichthe oxygen-excess mixed region includes oxygen, silicon, and at leastone of the metal elements included in the oxide semiconductor layer.

In the above structures, a protective insulating layer which covers theinsulating layer having defects may be provided.

Another embodiment of the present invention disclosed in thisspecification is a method for manufacturing a semiconductor device,including: forming a gate electrode layer and a gate insulating layerwhich covers the gate electrode layer over a substrate and introducingthe substrate into a process chamber under reduced pressure; introducinga sputtering gas from which hydrogen and moisture are removed whileremoving moisture remaining in the process chamber; forming an oxidesemiconductor layer over the gate insulating layer using a metal oxidetarget provided in the process chamber in which moisture is removed;forming a source electrode layer and a drain electrode layer over theoxide semiconductor layer; forming an oxygen-excess oxide insulatinglayer which is over the source electrode layer and the drain electrodelayer and which is in contact with the oxide semiconductor layer by asputtering method; forming an insulating layer having defects over theoxygen-excess oxide insulating layer by a sputtering method; and heatingthe substrate to make hydrogen or moisture included in the oxidesemiconductor layer move through the oxygen-excess oxide insulatinglayer and diffuse into the insulating layer having defects.

Another embodiment of the present invention disclosed in thisspecification is a method for manufacturing a semiconductor device,including: forming a gate electrode layer and a gate insulating layerwhich covers the gate electrode layer over a substrate and introducingthe substrate into a process chamber under reduced pressure; introducinga sputtering gas from which hydrogen and moisture are removed whileremoving moisture remaining in the process chamber; forming an oxidesemiconductor layer over the gate insulating layer using a metal oxidetarget provided in the process chamber in which moisture is removed;forming a source electrode layer and a drain electrode layer over theoxide semiconductor layer; forming an oxygen-excess mixed region whichis in contact with the oxide semiconductor layer, and an insulatinglayer having defects which is over the source electrode layer and thedrain electrode layer and which overlaps with the oxide semiconductorlayer with the oxygen-excess mixed region between the insulating layerhaving defects and the oxide semiconductor layer, by a sputteringmethod; and heating the substrate to make hydrogen or moisture includedin the oxide semiconductor layer move through the oxygen-excess mixedregion and diffuse into the insulating layer having defects.

In the above structures, the heat treatment for making impurities suchas hydrogen or moisture included in the oxide semiconductor layerdiffuse into the insulating layer having defects through theoxygen-excess mixed region or the oxygen-excess oxide insulating layermay be performed after or while a protective insulating layer is formedover the insulating layer having defects (at least over a portion of theinsulating layer having defects which overlaps with a channel formationregion in the oxide semiconductor layer). The heat treatment isperformed at 100° C. to 400° C. (or 150° C. to 400° C.).

In the above methods for manufacturing a semiconductor device, as thetarget for forming the oxide semiconductor film, a target including zincoxide as a main component can be used. Alternatively, metal oxideincluding indium, gallium, or zinc can be used as the target.

In the above methods for manufacturing a semiconductor device, theinsulating layer having defects may be a silicon oxide film. As a targetincluding silicon for forming the silicon oxide film, a silicon targetor a synthetic quartz target can be used.

With any one of the above structures, at least one of the above objectscan be achieved.

Note that a thin film of InMO₃(ZnO)_(m) (m>0) is used as an oxidesemiconductor layer and a thin film transistor is formed using the thinfilm as an oxide semiconductor layer. Note that M represents one or moremetal elements selected from Ga, Fe, Ni, Mn, and Co. For example, M maybe Ga or may include any of the above metal elements in addition to Ga;for example, M may be Ga and Ni or Ga and Fe. Moreover, in the aboveoxide semiconductor, in some cases, a transition metal element such asFe or Ni or oxide of the transition metal is included as an impurityelement in addition to a metal element included as M. In thisspecification, among the oxide semiconductor layers whose compositionformulas are represented by InMO₃(ZnO)_(m) (m>0), an oxide semiconductorwhich includes Ga as M is referred to as an In—Ga—Zn—O-based oxidesemiconductor, and a thin film of the In—Ga—Zn—O-based oxidesemiconductor is also referred to as an In—Ga—Zn—O-based film.

As metal oxide applicable to the oxide semiconductor layer, any of thefollowing oxide semiconductors can be applied besides the above:In—Sn—O-based, In—Sn—Zn—O-based, In—Al—Zn—O-based, Sn—Ga—Zn—O-based,Al—Ga—Zn—O-based, Sn—Al—Zn—O-based, In—Zn—O-based, Sn—Zn—O-based,Al—Zn—O-based, In—O-based, Sn—O-based, and Zn—O-based metal oxide.Silicon may be included in the oxide semiconductor layer formed usingthe above metal oxide.

The oxide semiconductor is preferably an oxide semiconductor containingIn, more preferably an oxide semiconductor containing In and Ga. Inorder to obtain an i-type (intrinsic) oxide semiconductor layer,dehydration or dehydrogenation is effective.

Further, an oxide conductive layer may be formed between the oxidesemiconductor layer and the source electrode layer and the drainelectrode layer. The oxide conductive layer and the metal layer forforming the source and drain electrode layers can be formedsuccessively.

Since a thin film transistor is easily broken due to static electricityor the like, a protective circuit for protecting the thin filmtransistor for a pixel portion is preferably provided over the samesubstrate as a gate line or a source line. The protective circuit ispreferably formed using a non-linear element including an oxidesemiconductor layer.

Note that the ordinal numbers such as first and second in thisspecification are used for convenience and do not denote the order ofsteps and the stacking order of layers. In addition, the ordinal numbersin this specification do not denote particular names which specify thepresent invention.

A semiconductor device including an oxide semiconductor with stableelectric characteristics can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1E illustrate a semiconductor device and a manufacturingmethod thereof.

FIGS. 2A to 2D illustrate a semiconductor device and a manufacturingmethod thereof.

FIGS. 3A to 3E illustrate a semiconductor device and a manufacturingmethod thereof.

FIGS. 4A to 4C illustrate a semiconductor device and a manufacturingmethod thereof.

FIGS. 5A to 5E illustrate a semiconductor device and a manufacturingmethod thereof.

FIGS. 6A to 6D illustrate a semiconductor device and a manufacturingmethod thereof.

FIG. 7 illustrates a semiconductor device.

FIGS. 8A to 8C illustrate a semiconductor device.

FIG. 9 is a pixel equivalent circuit diagram of a semiconductor device.

FIGS. 10A to 10C each illustrate a semiconductor device

FIGS. 11A and 11B illustrate a semiconductor device.

FIG. 12 illustrates a semiconductor device.

FIGS. 13A and 13B each illustrate an electronic device.

FIGS. 14A and 14B each illustrate an electronic device.

FIG. 15 illustrates an electronic device.

FIG. 16 illustrates an electronic device.

FIG. 17 illustrates electronic devices.

FIG. 18 illustrates a semiconductor device.

FIG. 19 is a longitudinal cross-sectional view of an inverted staggeredthin film transistor including an oxide semiconductor.

FIGS. 20A and 20B are energy band diagrams (schematic diagrams) alongA-A′ section illustrated in FIG. 19.

FIG. 21A shows a state in which a positive potential (+VG) is applied toa gate (G1), and FIG. 21B shows a state in which a negative potential(−VG) is applied to the gate (G1).

FIG. 22 shows a relation between the vacuum level and the work functionof a metal (φM), and between the vacuum level and the electron affinityof an oxide semiconductor (χ).

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. Note that thepresent invention is not limited to the following description and itwill be readily appreciated by those skilled in the art that modes anddetails can be modified in various ways. Accordingly, the presentinvention should not be construed as being limited to the description ofthe embodiments to be given below.

Embodiment 1

In this embodiment, an embodiment of a semiconductor device and amanufacturing method thereof will be described with reference to FIGS.1A to 1E. The semiconductor device described in this embodiment is athin film transistor.

FIGS. 1A to 1E illustrate an example of a cross-sectional structure of asemiconductor device. A thin film transistor 110 illustrated in FIGS. 1Ato 1E has one type of a bottom-gate structure and is also referred to asan inverted staggered thin film transistor.

The thin film transistor 110 illustrated in FIGS. 1A to 1E includes,over a substrate 100 having an insulating surface, a gate electrodelayer 111, a gate insulating layer 102, an oxide semiconductor layer112, an oxygen-excess mixed region 119, a source electrode layer 115 a,and a drain electrode layer 115 b. In addition, an insulating layer 116having defects which covers the thin film transistor 110 and overlapswith the oxide semiconductor layer 112 with the oxygen-excess mixedregion 119 therebetween is provided, and a protective insulating layer103 is additionally provided over the insulating layer 116 havingdefects.

Since the oxygen-excess mixed region 119 and the insulating layer 116having defects have a high binding energy to hydrogen or moisture (ahydrogen atom or a compound including a hydrogen atom such as H₂O) andthese impurities are stabilized in the oxygen-excess mixed region 119and the insulating layer 116 having defects, these impurities can bediffused from the oxide semiconductor layer into the oxygen-excess mixedregion 119 and the insulating layer 116 having defects, whereby theseimpurities can be removed from the oxide semiconductor layer. Further,the oxygen-excess mixed region 119 functions as a barrier layer againstimpurities which have been diffused into the insulating layer 116 havingdefects to prevent the impurities from entering the oxide semiconductorlayer 112 again; thus, the impurity concentration of the oxidesemiconductor layer 112 can be kept low. Accordingly, the thin filmtransistor 110 including the oxide semiconductor layer 112 in whichimpurities such as hydrogen, moisture, hydroxyl, or hydride (alsoreferred to as a hydrogen compound) which cause variation are reduced isa highly reliable thin film transistor with stable electriccharacteristics.

FIG. 19 is a longitudinal cross-sectional view of an inverted staggeredthin film transistor including an oxide semiconductor. An oxidesemiconductor layer (OS) is provided over a gate electrode (GE1) with agate insulating film (GI) interposed therebetween, and a sourceelectrode (S) and a drain electrode (D) are provided thereover.

FIGS. 20A and 20B are energy band diagrams (schematic diagrams) alongA-A′ section illustrated in FIG. 19. FIG. 20A illustrates the case wherethe potential of voltage applied to the source is equal to the potentialof voltage applied to the drain (VD=0 V), and FIG. 20B illustrates thecase where a positive potential with respect to the source is applied tothe drain (VD>0).

FIGS. 21A and 21B are energy band diagrams (schematic diagrams) alongB-B′ section illustrated in FIG. 19. FIG. 21A illustrates an on state inwhich a positive potential (+VG) is applied to the gate (G1) andcarriers (electrons) flow between the source and the drain. FIG. 21Billustrates an off state in which a negative potential (−V_(G)) isapplied to the gate (G1) and minority carriers do not flow.

FIG. 22 shows a relation between the vacuum level and the work functionof a metal (φM), and between the vacuum level and the electron affinityof an oxide semiconductor (χ).

Because electrons in metal are degenerated under room temperature, aFermi level is located in a conduction band. In contrast, a conventionaloxide semiconductor is generally of n-type, and Fermi level (Ef) in thatcase is located closer to the conduction band and is away from theintrinsic Fermi level (Ei) that is located in the middle of the bandgap. Note that it is known that one of a factor that part of hydrogen isa donor which donates an electron in an oxide semiconductor, aconventional oxide semiconductor to be an n-type oxide semiconductor.Note that it is known that one of the factors which make a conventionaloxide semiconductor to be an n-type oxide semiconductor is that part ofhydrogen in an oxide semiconductor becomes a donor which donates anelectron.

On the other hand, an oxide semiconductor according to the presentinvention is an intrinsic (i-type) or a substantially intrinsic oxidesemiconductor which is obtained by removing hydrogen that is an n-typeimpurity from an oxide semiconductor and highly purifying the oxidesemiconductor so that impurities that are not main components of theoxide semiconductor is prevented from being contained therein as much aspossible. In other words, a highly purified i-type (intrinsic)semiconductor or a semiconductor close thereto is obtained not by addingan impurity but by removing impurities such as hydrogen or water as muchas possible. This enables Fermi level (Ef) to be at the same level or tobe substantially the same level as the intrinsic Fermi level (Ei).

It is said that the electron affinity (χ) of an oxide semiconductor is4.3 eV in the case where the band gap (Eg) thereof is 3.15 eV. The workfunction of titanium (Ti) used for forming the source and drainelectrodes is substantially equal to the electron affinity (χ) of theoxide semiconductor. In that case, a Schottky barrier for electrons isnot formed at an interface between the metal and the oxidesemiconductor.

In other words, in the case where the work function of metal (φM) andthe electron affinity (χ) of the oxide semiconductor are equal to eachother and the metal and the oxide semiconductor are in contact with eachother, an energy band diagram (a schematic diagram) as illustrated inFIG. 20A is obtained.

In FIG. 20B, a black circle () represents an electron. When a positivepotential is applied to the drain, the electron is injected into theoxide semiconductor over the barrier (h) and flows toward the drain. Inthat case, the height of the barrier (h) changes depending on the gatevoltage and the drain voltage; in the case where positive drain voltageis applied, the height of the barrier (h) is smaller than the height ofthe barrier in FIG. 20A where no voltage is applied, i.e., ½ of the bandgap (Eg).

In this case, as shown in FIG. 21A, the electron moves along the lowestpart of the oxide semiconductor, which is energetically stable, at aninterface between the gate insulating film and the highly-purified oxidesemiconductor.

In FIG. 21B, when a negative potential (reverse bias) is applied to thegate (G1), the number of holes that are minority carriers issubstantially zero; thus, the current value becomes a value extremelyclose to zero.

For example, even when the thin film transistor has a channel width W of1×10⁴ μm and a channel length of 3 μm, an off current of 10⁻¹³ A orlower and a subthreshold value (S value) of 0.1 V/dec. (the thickness ofthe gate insulating film: 100 nm) can be obtained.

As described above, the oxide semiconductor is highly purified so thatimpurities that are not main components of the oxide semiconductor isprevented from being contained therein as much as possible, wherebyfavorable operation of the thin film transistor can be obtained.

Although the thin film transistor 110 is described as a single-gate thinfilm transistor, a multi-gate thin film transistor including a pluralityof channel formation regions can be formed if needed.

Hereinafter, a process for manufacturing the thin film transistor 110over the substrate 100 will be described with reference to FIGS. 1A to1E.

First, a conductive film is formed over the substrate 100 having aninsulating surface, and then the gate electrode layer 111 is formed by afirst photolithography step. It is preferable that an end portion of theformed gate electrode layer be tapered because coverage with a gateinsulating layer formed thereover is improved. Note that a resist maskmay be formed by an ink jetting method. The formation of the resist maskby an ink jetting method does not use a photomask; thus, manufacturingcost can be reduced.

Although there is no particular limitation on a substrate which can beused as the substrate 100 having an insulating surface, it is necessarythat the substrate have at least enough heat resistance to withstandheat treatment performed later. A glass substrate of barium borosilicateglass, aluminoborosilicate glass or the like can be used.

As a glass substrate, if the temperature of the heat treatment to beperformed later is high, a glass substrate whose strain point is 730° C.or higher is preferably used. As a glass substrate, a glass materialsuch as aluminosilicate glass, aluminoborosilicate glass, or bariumborosilicate glass is used, for example. Note that by containing alarger amount of barium oxide (BaO) than boron oxide, a glass substratewhich is heat-resistant and more practical can be obtained. Therefore, aglass substrate containing more BaO than B₂O₃ is preferably used.

Note that a substrate formed of an insulator such as a ceramicsubstrate, a quartz substrate, or a sapphire substrate may be usedinstead of the above glass substrate. Alternatively, crystallized glassor the like may be used. Further alternatively, a plastic substrate orthe like can be used as appropriate.

An insulating film functioning as a base film may be provided betweenthe substrate 100 and the gate electrode layer 111. The base film has afunction of preventing diffusion of impurity elements from the substrate100, and can be formed to have a single-layer or stacked-layer structureincluding one or more of a silicon nitride film, a silicon oxide film, asilicon nitride oxide film, and a silicon oxynitride film.

The gate electrode layer 111 can be formed to have a single-layer or astacked-layer structure using a metal material such as molybdenum,titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, orscandium, or an alloy material which includes any of these materials asits main component.

For example, as a two-layer structure of the gate electrode layer 111, atwo-layer structure in which a molybdenum layer is formed over analuminum layer, a two-layer structure in which a molybdenum layer isformed over a copper layer, a two-layer structure in which a titaniumnitride layer or a tantalum nitride layer is formed over a copper layer,or a two-layer structure in which a molybdenum layer is formed over atitanium nitride layer is preferable. As a three-layer structure, astacked-layer structure in which a tungsten layer or a tungsten nitridelayer, a layer of an alloy of aluminum and silicon or alloy of aluminumand titanium, and a titanium nitride layer or a titanium layer arestacked is preferable. Note that the gate electrode layer can be formedusing a light-transmitting conductive film. As an example of alight-transmitting conductive film, a light-transmitting conductiveoxide can be given.

Then, the gate insulating layer 102 is formed over the gate electrodelayer 111.

The gate insulating layer 102 can be formed to have a single-layer or astacked-layer structure including a silicon oxide layer, a siliconnitride layer, a silicon oxynitride layer, a silicon nitride oxidelayer, an aluminum oxide layer, an aluminum nitride layer, an aluminumoxynitride layer, an aluminum nitride oxide layer, or a hafnium oxidelayer by a plasma CVD method, a sputtering method, or the like. In orderto prevent the gate insulating layer 102 from including a large amountof hydrogen, the gate insulating layer 102 is preferably formed by asputtering method. When a silicon oxide film is formed by a sputteringmethod, a silicon target or a quartz target is used as a target andoxygen or a mixed gas of oxygen and argon is used as a sputtering gas.

The gate insulating layer 102 can have a stacked-layer structure inwhich a silicon nitride layer and a silicon oxide layer are stacked overthe gate electrode layer 111 in this order. For example, a gateinsulating layer having a thickness of 100 nm is formed in such a mannerthat a silicon nitride layer (SiN_(y) (y>0)) having a thickness of 50 nmto 200 nm inclusive is formed by a sputtering method as a first gateinsulating layer, and a silicon oxide layer (SiO_(x) (x>0)) having athickness of 5 nm to 300 nm inclusive is formed as a second gateinsulating layer over the first gate insulating layer. The thickness ofthe gate insulating layer may be set as appropriate depending on thedesired characteristics of the thin film transistor. The thickness maybe approximately 350 nm to 400 nm.

Further, in order that hydrogen, hydroxyl, and moisture may be containedin the gate insulating layer 102 and the oxide semiconductor film 120 aslittle as possible, it is preferable that the substrate 100 over whichthe gate electrode layer 111 is formed or the substrate 100 over whichlayers up to the gate insulating layer 102 are formed be preheated in apreheating chamber of a sputtering apparatus as pretreatment for filmformation so that impurities such as hydrogen and moisture adsorbed tothe substrate 100 is removed and exhausted. Note that the temperature ofthe preheating is 100° C. to 400° C. inclusive, preferably 150° C. to300° C. inclusive. As an evacuation means provided for the preheatingchamber, a cryopump is preferable. Note that this preheating treatmentcan be omitted. Further, this preheating may be similarly performed onthe substrate 100 over which layers up to the source electrode layer 115a and the drain electrode layer 115 b are formed, before the formationof the insulating layer 116 having defects.

Then, the oxide semiconductor film 120 having a thickness of 2 nm to 200nm inclusive is formed over the gate insulating layer 102 (see FIG. 1A).

Note that before the oxide semiconductor film 120 is formed by asputtering method, dust attached to a surface of the gate insulatinglayer 102 is preferably removed by reverse sputtering in which an argongas is introduced and plasma is generated. The reverse sputtering refersto a method in which, without application of voltage to a target side,an RF power source is used for application of voltage to a substrateside in an argon atmosphere in order to generate plasma in the vicinityof the substrate to modify a surface. Note that instead of an argonatmosphere, nitrogen, helium, oxygen, or the like may be used.

The oxide semiconductor film 120 is formed by a sputtering method. Asthe oxide semiconductor film 120, an In—Ga—Zn—O-based film, anIn—Sn—Zn—O-based oxide semiconductor film, an In—Al—Zn—O-based oxidesemiconductor film, an Sn—Ga—Zn—O-based oxide semiconductor film, anAl—Ga—Zn—O-based oxide semiconductor film, an Sn—Al—Zn—O-based oxidesemiconductor film, an In—Zn—O-based oxide semiconductor film, anSn—Zn—O-based oxide semiconductor film, an Al—Zn—O-based oxidesemiconductor film, an In—O-based oxide semiconductor film, anSn—O-based oxide semiconductor film, or a Zn—O-based oxide semiconductorfilm is used. In this embodiment, the oxide semiconductor film 120 isformed by a sputtering method using an In—Ga—Zn—O-based metal oxidetarget. Further, the oxide semiconductor film 120 can be formed by asputtering method in a rare gas (typically argon) atmosphere, an oxygenatmosphere, or an atmosphere of a rare gas (typically argon) and oxygen.In the case of film formation by a sputtering method, a target includingSiO₂ at 2 wt % to 10 wt % inclusive may be used.

A sputtering gas used in the formation of the oxide semiconductor film120 is preferably a high-purity gas in which impurities such ashydrogen, water, hydroxyl, or hydride are reduced to such a degree thatthe concentration thereof can be expressed by the unit ppm or ppb.

As a target for forming the oxide semiconductor film 120 by a sputteringmethod, a metal oxide target including zinc oxide as its main componentcan be used. Another example of a metal oxide target which can be usedis a metal oxide target including In, Ga, and Zn (with a compositionratio of In₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio]. As the metal oxide targetincluding In, Ga, and Zn, a target having a composition ratio ofIn₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio] or a target having a compositionratio of In₂O₃:Ga₂O₃:ZnO=1:1:4 [molar ratio] can be used. The fillingfactor of the metal oxide target is 90% to 100% inclusive, andpreferably 95% to 99.9% inclusive. With the use of a metal oxide targetwith high filling factor, the formed oxide semiconductor film has highdensity.

The substrate is placed in a process chamber under reduced pressure, andis heated to a temperature lower than 400° C. While moisture remainingin the process chamber is removed, a sputtering gas from which hydrogenand moisture are removed is introduced to form the oxide semiconductorfilm 120 over the substrate 100 using metal oxide as a target. In orderto remove moisture remaining in the process chamber, a capture-typevacuum pump is preferably used. For example, a cryopump, an ion pump, ora titanium sublimation pump is preferably used. An evacuation means maybe a turbo pump provided with a cold trap. In the film-formation chamberwhich is evacuated with the cryopump, hydrogen atoms, a compoundincluding a hydrogen atom such as H₂O, and a compound including a carbonatom, for example, are exhausted. Accordingly, the concentration ofimpurities included in the oxide semiconductor film formed in thisfilm-formation chamber can be reduced.

For the formation of the oxide semiconductor film, not only in theprocess chamber for forming the oxide semiconductor film but also in theprocess chamber for steps before and after formation of films in contactwith the oxide semiconductor film and steps before and after theformation of the oxide semiconductor film, an evacuation means such as acryopump is preferably used in order to prevent impurities such asmoisture remaining in the process chamber from being mixed into theoxide semiconductor film.

As an example of film formation conditions, the following condition isemployed: the distance between the substrate and the target is 100 mm,the pressure is 0.6 Pa, the direct current (DC) power supply is 0.5 kW,and the atmosphere is an oxygen atmosphere (the proportion of the oxygenflow is 100%). A pulse direct current (DC) power supply is preferablebecause powder substances (also referred to as particles or dust)generated in the film formation can be reduced and the film thicknesscan be made uniform. The oxide semiconductor film preferably has athickness of 5 nm to 30 nm inclusive. Note that an appropriate thicknessof the oxide semiconductor film varies depending on the material;therefore, the thickness may be determined depending on the material.

By forming the oxide semiconductor film 120 as described above by asputtering method, an oxide semiconductor film with a low hydrogenconcentration can be obtained. The hydrogen concentration given in thisspecification is a quantified result obtained by secondary ion massspectrometry (SIMS).

Examples of a sputtering method include an RF sputtering method in whicha high-frequency power source is used for a sputtering power supply, aDC sputtering method in which a DC power source is used, and a pulsed DCsputtering method in which a bias is applied in a pulsed manner. An RFsputtering method is mainly used in the case where an insulating film isformed, and a DC sputtering method is mainly used in the case where ametal film is formed.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can beformed to be stacked in the same chamber, or a film can be formed byelectric discharge of plural kinds of materials at the same time in thesame chamber.

In addition, there are a sputtering apparatus provided with a magnetsystem inside the chamber and used for a magnetron sputtering method,and a sputtering apparatus used for an ECR sputtering method in whichplasma generated with the use of microwaves is used without using glowdischarge.

Furthermore, as a film-formation method using a sputtering method, thereare also a reactive sputtering method in which a target substance and asputtering gas component are chemically reacted with each other duringfilm formation to form a thin compound film thereof, and a biassputtering method in which voltage is also applied to a substrate duringfilm formation.

Then, the oxide semiconductor film is processed into an island-shapedoxide semiconductor layer 121 by a second photolithography step (seeFIG. 1B). Note that a resist mask for forming the island-shaped oxidesemiconductor layer 121 may be formed by an ink jetting method. Theformation of the resist mask by an ink jetting method does not use aphotomask; thus, manufacturing cost can be reduced.

In the case of forming a contact hole in the gate insulating layer 102,the step can be performed at the time of the formation of the oxidesemiconductor layer 121.

The etching of the oxide semiconductor film 120 may be performed by dryetching, dry etching, or both wet etching and dry etching.

As an etching gas used for dry etching, a gas including chlorine (achlorine-based gas such as chlorine (Cl₂), boron chloride (BCl₃),silicon chloride (SiCl₄), or carbon tetrachloride (CCl₄)) is preferablyused.

Further, a gas including fluorine (a fluorine-based gas such as carbontetrafluoride (CF₄), sulfur fluoride (SF₆), nitrogen fluoride (NF₃), ortrifluoromethane (CHF₃)), hydrogen bromide (HBr), oxygen (O₂), any ofthese gases to which a rare gas such as helium (He) or argon (Ar) isadded, or the like can be used.

As the dry etching method, a parallel plate reactive ion etching (RIE)method or an inductively coupled plasma (ICP) etching method can beused. In order to etch the film into a desired shape, the etchingconditions (the amount of electric power applied to a coil-shapedelectrode, the amount of electric power applied to an electrode on asubstrate side, the temperature of the electrode on the substrate side,or the like) is adjusted as appropriate.

As an etchant used for wet etching, for example, a solution obtained bymixing phosphoric acid, acetic acid, and nitric acid, and an ammoniaperoxide mixture (31 wt % hydrogen peroxide water:28 wt % ammoniawater:water=5:2:2), or the like can be used. Further, ITO-07N (producedby KANTO CHEMICAL CO., INC.) may also be used.

The etchant after the wet etching is removed together with the materialwhich is etched off by cleaning. The waste liquid including the etchantand the material which is etched off may be purified and the materialmay be reused. Materials such as indium included in the oxidesemiconductor layer are collected from the waste liquid after theetching and reused, so that resources can be effectively used andmanufacturing cost can be reduced.

The etching conditions (such as an etchant, etching time, andtemperature) are adjusted as appropriate depending on the material sothat the film can be etched into the desired shape.

Note that it is preferable that reverse sputtering be performed beforeformation of a conductive film in the subsequent step in order to removea resist residue or the like attached on the surfaces of the oxidesemiconductor layer 121 and the gate insulating layer 102.

Then, a conductive film is formed over the gate insulating layer 102 andthe oxide semiconductor layer 121. The conductive film may be formed bya sputtering method or a vacuum evaporation method. As a material of thesecond conductive film, an element selected from aluminum (Al), chromium(Cr), copper (Cu), tantalum (Ta), titanium (Ti), molybdenum (Mo), andtungsten (W), an alloy containing any of these elements as a component,an alloy containing any of these the elements in combination, or thelike can be given. Further, one or more materials selected frommanganese (Mn), magnesium (Mg), zirconium (Zr), beryllium (Be), andthorium (Th) may be used. Further, the metal conductive film may have asingle-layer structure or a stacked-layer structure of two or morelayers. For example, a single-layer structure of an aluminum filmincluding silicon, a two-layer structure in which a titanium film isstacked over an aluminum film, a three-layer structure in which atitanium film, an aluminum film, and a titanium film are stacked in thisorder can be given. Alternatively, a film, an alloy film, or a nitridefilm which contains aluminum (Al) and one or a plurality of elementsselected from titanium (Ti), tantalum (Ta), tungsten (W), molybdenum(Mo), chromium (Cr), neodymium (Nd), and scandium (Sc) may be used.

By a third photolithography step, a resist mask is formed over theconductive film, and etching is selectively performed; thus, a sourceelectrode layer 115 a and a drain electrode layer 115 b are formed.Then, the resist mask is removed (see FIG. 1C).

Ultraviolet, a KrF laser light, or an ArF laser light is used for lightexposure for forming the resist mask in the third photolithography step.A channel length L of the thin film transistor to be formed laterdepends on a width of an interval between a bottom portion of the sourceelectrode layer and a bottom portion of the drain electrode layer whichare adjacent to each other over the oxide semiconductor layer 121. Notethat when light exposure is performed for the channel length L of lessthan 25 nm, extreme ultraviolet with extremely short wavelengths ofseveral nanometers to several tens of nanometers is used for lightexposure for forming the resist mask in the third photolithography step.Light exposure with extreme ultraviolet leads to a high resolution and alarge depth of field. Accordingly, the channel length L of the thin filmtransistor to be formed later can be set to 10 nm to 1000 nm inclusive.Thus, the operation speed of a circuit can be increased, and low powerconsumption can be achieved due to extremely small off-state current.

Note that each material and etching conditions are adjusted asappropriate so that the oxide semiconductor layer 121 should not removedby etching of the conductive film.

In this embodiment, a Ti film is used as the conductive film, anIn—Ga—Zn—O-based is used as the oxide semiconductor layer 121, and anammonia hydrogen peroxide (a mixture of ammonia, water, and hydrogenperoxide) is used as an etchant.

Note that in the third photolithography step, the oxide semiconductorlayer 121 may be partly etched in some cases to be an oxidesemiconductor layer having a groove (a depression portion). A resistmask for forming the source electrode layer 115 a and the drainelectrode layer 115 b may be formed by an ink jetting method. Theformation of the resist mask by an ink jetting method does not use aphotomask; thus, manufacturing cost can be reduced.

Further, in order to reduce the number of photomasks used in thephotolithography steps and reduce the number of photolithography steps,an etching step may be performed with the use of a multi-tone mask whichis a light-exposure mask through which light is transmitted to have aplurality of intensities. A resist mask formed using a multi-tone maskhas a plurality of thicknesses and further can be changed in shape bybeing etched, and thus can be used in a plurality of etching steps toprovide different patterns. Therefore, a resist mask corresponding to atleast two kinds of different patterns can be formed using one multi-tonemask. Thus, the number of light-exposure masks can be reduced and thenumber of corresponding photolithography steps can also be reduced,whereby simplification of the process can be realized.

Plasma treatment using a gas such as N₂O, N₂, or Ar may be performed toremove adsorbed water or the like of an exposed surface of the oxidesemiconductor layer. Plasma treatment may be performed using a mixed gasof oxygen and argon.

Then, the insulating layer 116 having defects is formed over the oxidesemiconductor layer 121 without exposure to air. In the formation of theinsulating layer 116 having defects, the oxygen-excess mixed region 119is formed between the oxide semiconductor layer 121 and the insulatinglayer 116 having defects (see FIG. 1D). In this embodiment, theinsulating layer 116 having defects is formed to overlap with the oxidesemiconductor layer 121, and the oxygen-excess mixed region 119 isprovided therebetween in a region where the oxide semiconductor layer121 does not overlap with the source electrode layer 115 a or the drainelectrode layer 115 b.

The mixed region is a mixed region of materials included in the oxidesemiconductor layer and in the overlying insulating layer havingdefects. By providing the mixed region, an interface between the oxidesemiconductor layer and the insulating layer having defects is notclearly defined; thus, diffusion of hydrogen from the oxidesemiconductor layer into the insulating layer having defects isfacilitated. For example, when a silicon oxide layer is used as theinsulating layer having defects, the mixed region includes oxygen,silicon, and at least one of the metal elements included in the oxidesemiconductor layer.

As in this embodiment, in the case where a silicon oxide is used for theinsulating layer 116 having defects and an In—Ga—Zn—O-based film is usedas the oxide semiconductor, the mixed region 119 includes oxygen,silicon, and at least one metal element selected from In, Ga, and Zn.The metal in the oxide semiconductor can exist in a variety of states inthe mixed region 119; referring to the metal included in the oxidesemiconductor as M, the metal in the mixed region 119 can be expressedas M-OH, M-H, M-O—Si—H, and M-O—Si—OH, specifically, Zn—H, Zn—OH, andthe like.

The mixed region 119 may have a thickness of 0.1 nm to 30 nm(preferably, 2 nm to 10 nm). The thickness of the mixed region 119 canbe controlled by the film formation conditions of the sputtering methodat the time of forming the insulating layer 116 having defects. If thepower supply is set higher and the distance between the substrate andthe target is set shorter in the sputtering method, the mixed region 119can be formed thicker. When the sputtering method is conducted withhigher power supply, water or the like adsorbed on a surface of theoxide semiconductor layer 121 can be removed.

The provision of the mixed region 119 between the oxide semiconductorlayer 121 and the insulating layer 116 having defects facilitatesdiffusion of hydrogen atoms, a compound including a hydrogen atom suchas H₂O, a compound including a carbon atom, and the like which areincluded in the oxide semiconductor layer 121 into the insulating layer116 having defects.

The mixed region 119 needs to include excess oxygen and therefore isformed using a sputtering gas which includes much oxygen in order toprovide an oxygen-excess region, and after the formation of the mixedregion 119, the amount of oxygen in the sputtering gas may be adjustedfor the formation of the insulating layer 116 having defects.

Instead of a silicon oxide layer, a silicon oxynitride layer, analuminum oxide layer, an aluminum oxynitride layer, or the like can beused as the insulating layer 116 having defects. Further, a siliconnitride layer, a silicon nitride oxide layer, an aluminum nitride layer,an aluminum nitride oxide layer, or the like may be used as theinsulating layer 116 having defects.

In this embodiment, to form an oxygen-excess mixed region and a siliconoxide layer, the substrate 100 over which layers up to the island-shapedoxide semiconductor layer 121, the source electrode layer 115 a, and thedrain electrode layer 115 b are formed is heated to room temperature ora temperature lower than 100° C., a sputtering gas including high-purityoxygen from which hydrogen and moisture are removed is introduced, and asilicon target is used.

A sputtering gas used in the formation of the insulating layer 116having defects is preferably a high-purity gas in which impurities suchas hydrogen, water, hydroxyl, or hydride are reduced to such a degreethat the concentration thereof can be expressed by the unit ppm or ppb.

For example, a silicon oxide film is formed by a pulsed DC sputteringmethod under the following condition: a boron-doped silicon target whichhas a purity of 6N (the resistivity is 0.01 Ωcm) is used; the distancebetween the substrate and the target (T-S distance) is 89 mm; thepressure is 0.4 Pa, the direct-current (DC) power source is 6 kW, andthe atmosphere is oxygen (the proportion of the oxygen flow is 100%).The film thickness is 300 nm. Note that instead of a silicon target,quartz (preferably, synthetic quartz) can be used as the target forforming the silicon oxide film. As a sputtering gas, an oxygen gas or amixed gas of oxygen and argon is used.

It is preferable that the mixed region 119 and the insulating layer 116having defects be formed while moisture remaining in the process chamberis removed so that hydrogen, hydroxyl, or moisture should not beincluded in the oxide semiconductor layer 121, the mixed region 119, orthe insulating layer 116 having defects.

Note that the mixed region 119 may be formed using silicon oxynitride,aluminum oxide, aluminum oxynitride, or the like, instead of siliconoxide.

Then, heat treatment is performed at 100° C. to 400° C., in a statewhere the insulating layer 116 having defects and the oxidesemiconductor layer 121 are in contact with each other with theoxygen-excess mixed region 119 therebetween. This heat treatment candiffuse hydrogen or moisture included in the oxide semiconductor layer121 into the oxygen-excess mixed region 119 and the insulating layer 116having defects. Since the oxygen-excess mixed region 119 is providedbetween the insulating layer 116 having defects and the oxidesemiconductor layer 121, impurities such as hydrogen, hydroxyl, ormoisture included in the island-shaped oxide semiconductor layer 121 arediffused from the oxide semiconductor layer 121 into the oxygen-excessmixed region 119 or into the insulating layer 116 having defects throughthe oxygen-excess mixed region 119.

The mixed region 119 which is provided between the oxide semiconductorlayer 121 and the insulating layer 116 having defects includes excessoxygen, and thus has many oxygen dangling bonds as defects and has highbinding energy to impurities such as hydrogen, moisture, hydroxyl, orhydride. The provision of the oxygen-excess mixed region 119 facilitatesdiffusion and movement of impurities such as hydrogen, moisture,hydroxyl, or hydride included in the oxide semiconductor layer 121 intothe insulating layer 116 having defects.

In addition, when the impurities which have been removed from the oxidesemiconductor layer 121 and diffused into the insulating layer 116having defects move back toward the oxide semiconductor layer 121, theoxygen-excess mixed region 119 functions as a protective layer (abarrier layer) which is bound to and stabilizes the impurities so as toprevent the impurities from entering the oxide semiconductor layer 121.

As described above, by removing impurities such as hydrogen, moisture,hydroxyl, or hydride which cause variation from the oxide semiconductorlayer 121, the oxide semiconductor layer 112 with reduced impurities canbe provided. Further, the oxygen-excess mixed region 119 which functionsas a barrier layer prevents the impurities which have been diffused intothe insulating layer 116 having defects from entering the oxidesemiconductor layer 112 again; thus, the impurity concentration of theoxide semiconductor layer 112 can be kept low.

An oxygen-excess mixed region or an oxygen-excess oxide insulatinglayer, which is provided between an oxide semiconductor layer and aninsulating layer having defects, includes excess oxygen, and thus hasmany oxygen dangling bonds as defects. Concerning diffusion of hydrogenfrom the oxide semiconductor layer into such an insulating layer havingdefects, it was calculated that in which of an oxide semiconductor layer(amorphous IGZO) and an insulating layer having defects (amorphousSiO_(x)), hydrogen atom are more likely to exist.

A binding energy E_bind of a hydrogen atom was defined as follows, sothat stability of the hydrogen atom in an environment was evaluated.

E_bind={E(original structure)+E(H)}−E(structure with H)

The larger this bound E_bind becomes, the more likely the hydrogen atomis to exist. E(original structure), E(H), and E(structure with H)respectively represent energy of the original structure, energy of thehydrogen atom, and energy of the structure with H. The binding energy offour samples was calculated: amorphous IGZO, amorphous SiO₂ withoutdangling bonds (hereinafter abbreviated to DB), and two kinds ofamorphous SiO_(x) with DB.

For calculation, CASTEP, which is a program for a density functionaltheory, was used. As a method for the density functional theory, a planewave basis pseudopotential method was used. As a functional, LDA wasused. Cut-off energy was 300 eV. A 2×2×2 grid K-point grid was used.

The calculated structures are described below. First, the originalstructure is described. A unit cell of amorphous IGZO includes 84 atomsin total: 12 In atoms, 12 Ga atoms, 12 Zn atoms, and 48 O atoms. A unitcell of amorphous SiO₂ without DB includes 48 atoms in total: 16 Siatoms and 32 O atoms. Amorphous SiO_(x) with DB (1) has such a structurein which one O atom is removed from the amorphous SiO₂ without DB andone Si atom which has been bonded to the O atom is bonded to H; that is,it includes 48 atoms in total: 16 Si atoms, 31 O atoms, and 1 H atom.Amorphous SiO_(x) with DB (2) has such a structure in which one Si atomis removed from the amorphous SiO₂ without DB and three O atoms whichhave been bonded to the Si atom are each bonded to H atoms; that is, itincludes 50 atoms in total: 15 Si atoms, 32 O atoms, and 3 H atoms. Thestructure with H is a structure in which H was attached to each of theabove four structures. Note that H was attached to an O atom in theamorphous IGZO, to a Si atom in the amorphous SiO₂ without DB, and to anatom that has DB in the amorphous SiO_(x) with DB. The structure inwhich H was calculated includes one H atom in a unit cell. Note that thecell size of each structure is shown in Table 1.

TABLE 1 cell size (nm) structure angle amorphous IGZO 1.0197 × 1.0197 ×1.0197 α = β = γ = 90° amorphous SiO₂ without DB 0.9127 × 0.9127 ×0.9127 amorphous SiOx(1) with DB α = β = γ = 90° amorphous SiOx(2) withDB Hydrogen atom 1.0000 × 1.0000 × 1.0000 α = β = γ = 90°

Calculation results are shown in Table 2.

TABLE 2 energy of energy of structure with original energy of H atom ina structure in a hydrogen binding unit cell (eV) unit cell (eV) atom(eV) energy(eV) amorphous −84951.3359 −84935.6442 −13.0015 2.69 IGZOamorphous −15783.8101 −15770.6279 −13.0015 0.18 SiO2 without DBamorphous −15363.1459 −15345.6884 −13.0015 4.46 SiOx(1) with DBamorphous −15722.2053 −15702.5905 −13.0015 6.61 SiOx(2) with DB

From the above, amorphous SiO_(x) with DB (2) having a structure inwhich Si is removed from the amorphous SiO₂ without DB and three O atomswhich have been bonded to the Si are each bonded to H has the highestbinding energy, followed by SiO_(x) (1) having a structure in which oneO atom is removed from the amorphous SiO₂ without DB and one Si atomwhich has been bonded to the one O atom is bonded to H, amorphous IGZO,and amorphous SiO₂ without DB having the lowest binding energy.Therefore, hydrogen becomes the most stable when being bonded to DB inamorphous SiO_(x) having DB which is caused by excess oxygen.

As a result, the following process can be assumed. There is a largeamount of DBs in amorphous SiO_(x). A hydrogen atom at the vicinity ofthe interface between amorphous IGZO and amorphous SiO_(x) becomesstable by being bonded to the DB in the amorphous SiO_(x). Thus, thehydrogen atom in the amorphous IGZO moves to the DB in the amorphousSiO_(x).

Further, from the fact that the amorphous SiO_(x) with DB (2) having thestructure in which dangling bonds are formed by removal of Si has ahigher binding energy than the SiO_(x) with DB (1) having the structurein which a dangling bond is formed by removal of O, hydrogen atoms inSiO_(x) are more stable when being bonded to O.

If the insulating layer having defects is an insulating layer havingmany oxygen dangling bonds as defects, its binding energy to hydrogen ishigh; accordingly, more hydrogen atoms or more impurities includinghydrogen can be diffused from the oxide semiconductor layer into theinsulating layer having defects. Therefore, the mixed region or theoxide insulating layer which is in contact with the oxide semiconductorlayer preferably includes excess oxygen, and is preferably expressed bySiO_(2+x) where x is equal to or greater than 0 and less than 3.

Through the above process, the thin film transistor 110 including theoxide semiconductor layer 112 in which the concentration of hydrogen andhydride is reduced can be formed (see FIG. 1E). By reducing theconcentration of the impurities such as hydrogen or moisture, generationof parasitic channel on the back channel side, i.e., in a superficialportion of the oxide semiconductor layer can be suppressed.

In the thin film transistor 110, a channel formation region can beformed in the oxide semiconductor layer in which the hydrogen is setequal to or less than 5×10¹⁹/cm³, preferably equal to or less than5×10¹⁸/cm³, and more preferably equal to or less than 5×10¹⁷/cm³;hydrogen or O—H group in the oxide semiconductor is removed; and thecarrier concentration is equal to or less than 5×10¹⁴/cm³, preferablyequal to or less than 5×10¹²/cm³.

The energy gap of the oxide semiconductor is set to be equal to orgreater than 2 eV, preferably equal to or greater than 2.5 eV, morepreferably equal to or greater than 3 eV to reduce as much impurities,such as hydrogen which form donors, as possible, and the carrierconcentration of the oxide semiconductor is set to equal to or less than1×10¹⁴/cm³, preferably equal to or less than 1×10¹²/cm³.

When the thus purified oxide semiconductor is used for a channelformation region of the thin film transistor 110, even in the case wherethe channel width is 10 mm, the drain current of equal to or less than1×10⁻¹³ A is obtained at drain voltages of 1 V and 10 V and gatevoltages in the range of −5 V to −20 V.

As described above, by removing remaining moisture in the reactionatmosphere for the formation of the oxide semiconductor film, theconcentration of hydrogen and hydride in the oxide semiconductor filmcan be reduced. In addition, by providing the insulating layer havingdefects over the oxide semiconductor layer with the oxygen-excess mixedregion therebetween, impurities such as hydrogen or moisture in theoxide semiconductor layer are diffused into the insulating layer havingdefects, whereby the concentration of hydrogen and hydride in the oxidesemiconductor layer can be reduced. Accordingly, the oxide semiconductorlayer can be stabilized.

A protective insulating layer may be provided over the insulating layerhaving defects. In this embodiment, the protective insulating layer 103is formed over the insulating layer 116 having defects. As theprotective insulating layer 103, a silicon nitride film, a siliconnitride oxide film, an aluminum nitride film, or the like can be used.

As the protective insulating layer 103, a silicon nitride film is formedby heating the substrate 100 over which layers up to the insulatinglayer 116 having defects are formed, to a temperature of 100° C. to 400°C.; introducing a sputtering gas including high-purity nitrogen fromwhich hydrogen and moisture are removed; and using a silicon target. Inthis step also, it is preferable that the protective insulating layer103 be formed while moisture remaining in the process chamber is removedas in the case of the insulating layer 116 having defects.

In the case of forming the protective insulating layer 103, if thesubstrate 100 is heated to a temperature of 100° C. to 400° C. at thetime of the formation of the protective insulating layer 103, impuritiessuch as hydrogen or moisture included in the oxide semiconductor layercan be diffused into the insulating layer 116 having defects. In thatcase, heat treatment is not necessarily performed after the formation ofthe insulating layer 116 having defects.

In the case where a silicon oxide layer is formed as the insulatinglayer 116 having defects and a silicon nitride layer is formed thereoveras the protective insulating layer 103, the silicon oxide layer and thesilicon nitride layer can be formed in the same process chamber using acommon silicon target. First, a sputtering gas including oxygen isintroduced and a silicon oxide layer is formed using a silicon targetplaced inside the process chamber, and then the sputtering gas isswitched to a sputtering gas including nitrogen and a silicon nitridelayer is formed using the same silicon target. Since the silicon oxidelayer and the silicon nitride layer can be formed in succession withoutexposure to air, impurities such as hydrogen or moisture can beprevented from being adsorbed on a surface of the silicon oxide layer.In that case, after the silicon oxide layer is formed as the insulatinglayer 116 having defects and the silicon nitride layer is formedthereover as the protective insulating layer 103, heat treatment (at atemperature of 100° C. to 400° C.) for diffusion of hydrogen or moisturein the oxide semiconductor layer into the insulating layer havingdefects is preferably performed.

After the formation of the protective insulating layer, heat treatmentmay be further performed at 100° C. to 200° C. inclusive in air for 1hour to 30 hours inclusive. This heat treatment may be performed at afixed heating temperature or the following change in the heatingtemperature may be conducted plural times repeatedly: the heatingtemperature is increased from room temperature to a temperature of 100°C. to 200° C. inclusive and then decreased to room temperature. Further,this heat treatment may be performed under reduced pressure before theformation of the protective insulating layer. Under reduced pressure,the heat treatment time can be shortened. With this heat treatment, anormally-off thin film transistor can be obtained. Accordingly, thereliability of the semiconductor device can be improved.

Even if impurities move back toward the oxide semiconductor layer 112due to the heat treatment after being diffused into the insulating layer116 having defects, the oxygen-excess mixed region 119 functioning as abarrier layer prevents the impurities from entering the oxidesemiconductor layer 112. Thus, the impurity concentration of the oxidesemiconductor layer 112 can be kept low.

The above process can be used for manufacturing a backplane (a substrateover which a thin film transistor is formed) of a liquid crystal displaypanel, an electroluminescent display panel, a display device usingelectronic ink, and the like. Since the above process can be performedat a temperature equal to or less than 400° C., the process can bepreferably applied to a manufacturing process using a glass substratehaving a side longer than 1 m and a thickness less than or equal to 1mm.

As described above, a highly reliable semiconductor device with stableelectric characteristics including a thin film transistor including anoxide semiconductor layer can be provided.

Embodiment 2

In this embodiment, another example of a thin film transistor which canbe applied to a semiconductor device disclosed in this specificationwill be described. The same portion as or a portion having a functionsimilar to those in the above embodiment can be formed in a mannersimilar to that described in the above embodiment, and also the stepssimilar to those in the above embodiment can be performed in a mannersimilar to that described in the above embodiment, and repetitivedescription is omitted. In addition, detailed description of the sameportions is not repeated.

FIGS. 2A to 2D illustrate an example of a cross-sectional structure of asemiconductor device. A thin film transistor 130 illustrated in FIGS. 2Ato 2D has one type of a bottom-gate structure and is also referred to asan inverted staggered thin film transistor.

The thin film transistor 130 illustrated in FIGS. 2A to 2D includes,over the substrate 100 having an insulating surface, the gate electrodelayer 111, the gate insulating layer 102, an oxide semiconductor layer132, the source electrode layer 115 a, and the drain electrode layer 115b. In addition, an oxygen-excess oxide insulating layer 139 which coversthe thin film transistor 130 and is in contact with the oxidesemiconductor layer 132 is provided, and the insulating layer 116 havingdefects is formed over the oxygen-excess oxide insulating layer 139. Inaddition, the protective insulating layer 103 is formed over theinsulating layer 116 having defects.

Since the oxygen-excess oxide insulating layer 139 and the insulatinglayer 116 having defects have a high binding energy to hydrogen ormoisture (a hydrogen atom or a compound including a hydrogen atom suchas H₂O) and these impurities are stabilized in the oxygen-excess oxideinsulating layer 139 and the insulating layer 116 having defects, theseimpurities can be diffused from the oxide semiconductor layer 132 intothe oxygen-excess oxide insulating layer 139 and the insulating layer116 having defects, whereby these impurities can be removed from theoxide semiconductor layer 132. Further, the oxygen-excess oxideinsulating layer 139 functions as a barrier layer against impuritieswhich have been diffused into the insulating layer 116 having defects toprevent the impurities from entering the oxide semiconductor layer 132again; thus, the impurity concentration of the oxide semiconductor layer132 can be kept low. Accordingly, the thin film transistor 130 includingthe oxide semiconductor layer 132 in which impurities such as hydrogen,moisture, hydroxyl, or hydride (also referred to as a hydrogen compound)which cause variation are reduced is a highly reliable thin filmtransistor with stable electric characteristics.

As the oxygen-excess oxide insulating layer 139, a silicon oxide layer(SiO_(2+x), where x is preferably equal to or greater than 0 and lessthan 3) can be used. The oxygen-excess oxide insulating layer 139 mayhave a thickness of 0.1 nm to 30 nm (preferably, 2 nm to 10 nm).

Although the thin film transistor 130 is described as a single-gate thinfilm transistor, a multi-gate thin film transistor including a pluralityof channel formation regions can be formed if needed.

Hereinafter, a process for manufacturing the thin film transistor 130over the substrate 100 will be described with reference to FIGS. 2A to2D.

First, a conductive film is formed over the substrate 100 having aninsulating surface, and then the gate electrode layer 111 is formed by afirst photolithography step.

Then, the gate insulating layer 102 is formed over the gate electrodelayer 111. The gate insulating layer 102 can have a stacked-layerstructure in which a silicon nitride layer and a silicon oxide layer arestacked over the gate electrode layer 111 in this order.

Then, an oxide semiconductor film is formed over the gate insulatinglayer 102 and processed into an island-shaped oxide semiconductor layer121 by a second photolithography step. In this embodiment, the oxidesemiconductor film is formed by a sputtering method using anIn—Ga—Zn—O-based metal oxide target.

Then, a conductive film is formed over the gate insulating layer 102 andthe oxide semiconductor layer 121. By a third photolithography step, aresist mask is formed over the conductive film, and selective etching isperformed; thus, the source electrode layer 115 a and the drainelectrode layer 115 b are formed. Then, the resist mask is removed (seeFIG. 2A).

Then, the oxygen-excess oxide insulating layer 139 is formed over thegate insulating layer 102, the oxide semiconductor layer 121, the sourceelectrode layer 115 a, and the drain electrode layer 115 b (see FIG.2B). In this embodiment, the oxygen-excess oxide insulating layer 139 isformed to be in contact with the oxide semiconductor layer 121 in aregion where the oxide semiconductor layer 121 does not overlap with thesource electrode layer 115 a or the drain electrode layer 115 b.

In this embodiment, to form a silicon oxide layer (SiO_(2+x), where x ispreferably equal to or greater than 0 and less than 3) as theoxygen-excess oxide insulating layer 139, the substrate 100 over whichlayers up to the source electrode layer 115 a and the drain electrodelayer 115 b are formed is heated to room temperature or a temperaturelower than 100° C., a sputtering gas including high-purity oxygen fromwhich hydrogen and moisture are removed is introduced, and a silicontarget is used. The thickness of the oxygen-excess oxide insulatinglayer 139 may be 0.1 nm to 30 nm (preferably, 2 nm to 10 nm).

A sputtering gas used in the formation of the oxygen-excess oxideinsulating layer 139 is preferably a high-purity gas in which impuritiessuch as hydrogen, water, hydroxyl, or hydride are reduced to such adegree that the concentration thereof can be expressed by the unit ppmor ppb.

For example, a silicon oxide layer is formed by a pulsed DC sputteringmethod under the following condition: a boron-doped silicon target whichhas a purity of 6N (the resistivity is 0.01 Ωcm) is used; the distancebetween the substrate and the target (S-T distance) is 89 mm; thepressure is 0.4 Pa, the direct-current (DC) power source is 6 kW, andthe atmosphere is oxygen (the proportion of the oxygen flow is 100%).Note that instead of a silicon target, quartz (preferably, syntheticquartz) can be used as the target for forming the silicon oxide layer.As a sputtering gas, an oxygen gas or a mixed gas of oxygen and argon isused.

Note that instead of a silicon oxide layer, a silicon oxynitride layer,an aluminum oxide layer, an aluminum oxynitride layer, or the like canbe used as the oxygen-excess oxide insulating layer 139.

Then the insulating layer 116 having defects is formed over theoxygen-excess oxide insulating layer 139 without exposure to air.

In this embodiment, to form the insulating layer 116 having defects, thesubstrate 100 over which layers up to the island-shaped oxidesemiconductor layer 121, the source electrode layer 115 a, the drainelectrode layer 115 b, and the oxygen-excess oxide insulating layer 139are formed is heated to room temperature or a temperature lower than100° C., a sputtering gas including high-purity oxygen from whichhydrogen and moisture are removed is introduced, and a silicon target isused. The oxygen-excess oxide insulating layer 139 and the insulatinglayer 116 having defects may be formed in the same process chamber usingthe same target.

A sputtering gas used in the formation of the insulating layer 116having defects is preferably a high-purity gas in which impurities suchas hydrogen, water, hydroxyl, or hydride are reduced to such a degreethat the concentration thereof can be expressed by the unit ppm or ppb.

It is preferable that oxygen-excess oxide insulating layer 139 and theinsulating layer 116 having defects be formed while moisture remainingin the process chamber where the oxygen-excess oxide insulating layer139 and the insulating layer 116 having defects are formed is removed sothat hydrogen, hydroxyl, or moisture should not be included in the oxidesemiconductor layer 121, the oxygen-excess oxide insulating layer 139,or the insulating layer 116 having defects.

The insulating layer 116 having defects may be any insulating layerhaving many defects and a silicon oxynitride layer, an aluminum oxidelayer, an aluminum oxynitride layer, or the like can be used instead ofthe silicon oxide layer. Further, a silicon nitride layer, a siliconnitride oxide layer, an aluminum nitride layer, an aluminum nitrideoxide layer, or the like may be used as the insulating layer 116 havingdefects.

Then, heat treatment is performed at 100° C. to 400° C., in a statewhere the insulating layer 116 having defects and the oxidesemiconductor layer 121 are in contact with each other with theoxygen-excess oxide insulating layer 139 therebetween. This heattreatment can diffuse hydrogen or moisture included in the oxidesemiconductor layer 121 into the oxygen-excess oxide insulating layer139 and the insulating layer 116 having defects. Since the oxygen-excessoxide insulating layer 139 is provided between the insulating layer 116having defects and the oxide semiconductor layer 121, impurities such ashydrogen, hydroxyl, or moisture included in the island-shaped oxidesemiconductor layer 121 are diffused from the oxide semiconductor layer121 into the oxygen-excess oxide insulating layer 139 or into theinsulating layer 116 having defects through the oxygen-excess oxideinsulating layer 139.

The oxide insulating layer 139, which is provided between the oxidesemiconductor layer 121 and the insulating layer 116 having defects,includes excess oxygen, and thus has many oxygen dangling bonds asdefects and has high binding energy to impurities such as hydrogen,moisture, hydroxyl, or hydride. The provision of the oxygen-excess oxideinsulating layer 139 facilitates diffusion and movement of impuritiessuch as hydrogen, moisture, hydroxyl, or hydride included in the oxidesemiconductor layer 121 into the insulating layer 116 having defects.

In addition, when the impurities which have been removed from the oxidesemiconductor layer 121 and diffused into the insulating layer 116having defects move back toward the oxide semiconductor layer, theoxygen-excess oxide insulating layer 139 functions as a protective layer(a barrier layer) which is bound to and stabilizes the impurities so asto prevent the impurities from entering the oxide semiconductor layer.

As described above, by removing impurities such as hydrogen, moisture,hydroxyl, or hydride which cause variation from the oxide semiconductorlayer, the oxide semiconductor layer 121 with reduced impurities can beprovided. Further, the oxygen-excess oxide insulating layer 139 whichfunctions as a barrier layer prevents the impurities which have beendiffused into the insulating layer 116 having defects from entering theoxide semiconductor layer 121 again; thus, the impurity concentration ofthe oxide semiconductor layer 121 can be kept low.

Then, the protective insulating layer 103 is formed over the insulatinglayer 116 having defects. As the protective insulating layer 103, asilicon nitride layer, a silicon nitride oxide layer, an aluminumnitride layer, an aluminum nitride oxide layer, or the like can be used.In this embodiment, as the protective insulating layer 103, a siliconnitride layer is formed by heating the substrate 100 over which layersup to the insulating layer 116 having defects are formed, to atemperature of 100° C. to 400° C.; introducing a sputtering gasincluding high-purity nitrogen from which hydrogen and moisture areremoved; and using a silicon target.

Impurities such as hydrogen or moisture are removed and theconcentration of those impurities is kept very low in the above process,whereby generation of a parasitic channel on the back channel side in asuperficial portion of the oxide semiconductor layer can be suppressed.

Thus, the thin film transistor 130 including the oxide semiconductorlayer 132 in which the concentration of impurities such as hydrogen andhydride is reduced can be formed (see FIG. 2D).

Even if impurities move back toward the oxide semiconductor layer due toheat treatment in the steps after being diffused into the insulatinglayer 116 having defects, the oxygen-excess oxide insulating layer 139functioning as a barrier layer prevents the impurities from entering theoxide semiconductor layer 132. Thus, the impurity concentration of theoxide semiconductor layer 132 can be kept low.

In the thin film transistor 130, a channel formation region can beformed in the oxide semiconductor layer in which the hydrogen is setequal to or less than 5×10¹⁹/cm³, preferably equal to or less than5×10¹⁸/cm³, and more preferably equal to or less than 5×10¹⁷/cm³;hydrogen or O—H group in the oxide semiconductor is removed; and thecarrier concentration is equal to or less than 5×10¹⁴/cm³, preferablyequal to or less than 5×10¹²/cm³.

The energy gap of the oxide semiconductor is set to be equal to orgreater than 2 eV, preferably equal to or greater than 2.5 eV, morepreferably equal to or greater than 3 eV to reduce as much impurities,such as hydrogen which form donors, as possible, and the carrierconcentration of the oxide semiconductor is set to equal to or less than1×10¹⁴/cm³, preferably equal to or less than 1×10¹²/cm³.

When the thus purified oxide semiconductor is used for a channelformation region of the thin film transistor 130, even in the case wherethe channel width is 10 mm, the drain current of equal to or less than1×10⁻¹³ A is obtained at drain voltages of 1 V and 10 V and gatevoltages in the range of −5 V to −20 V.

As described above, a highly reliable semiconductor device with stableelectric characteristics including a thin film transistor including anoxide semiconductor layer can be provided.

Embodiment 3

In this embodiment, another example of a thin film transistor which canbe applied to a semiconductor device disclosed in this specificationwill be described.

A semiconductor device and a manufacturing method thereof in thisembodiment will be described with reference to FIGS. 3A to 3E.

FIGS. 3A to 3E illustrate an example of a cross-sectional structure of asemiconductor device. A thin film transistor 160 illustrated in FIGS. 3Ato 3E has one type of a bottom-gate structure called achannel-protective structure (also referred to as a channel-stopstructure) and is also referred to as an inverted staggered thin filmtransistor.

Although the thin film transistor 160 is described as a single-gate thinfilm transistor, a multi-gate thin film transistor including a pluralityof channel formation regions can be formed if needed.

Hereinafter, a process for manufacturing the thin film transistor 160over the substrate 150 will be described with reference to FIGS. 3A to3D.

First, a conductive film is formed over the substrate 150 having aninsulating surface, and then the gate electrode layer 151 is formed by afirst photolithography step. Note that a resist mask may be formed by anink jetting method. The formation of the resist mask by an ink jettingmethod does not use a photomask; thus, manufacturing cost can bereduced.

The gate electrode layer 151 can be formed to have a single-layer or astacked-layer structure using a metal material such as molybdenum,titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, orscandium, or an alloy material which includes any of these materials asits main component.

Then, a gate insulating layer 152 is formed over the gate electrodelayer 151.

In this embodiment, a silicon oxynitride layer having a thickness of 100nm is formed by a plasma CVD method as the gate insulating layer 152.

Then, an oxide semiconductor film is formed over the gate insulatinglayer 152 and processed into an island-shaped oxide semiconductor layer171 by a second photolithography step. In this embodiment, the oxidesemiconductor film is formed by a sputtering method using anIn—Ga—Zn—O-based metal oxide target.

The substrate is placed in a process chamber under reduced pressure, andis heated to a temperature lower than 400° C. While moisture remainingin the process chamber is removed, a sputtering gas from which hydrogenand moisture are removed is introduced to form the oxide semiconductorfilm over the substrate 150 using metal oxide as a target. In order toremove moisture remaining in the process chamber, a capture-type vacuumpump is preferably used. For example, a cryopump, an ion pump, or atitanium sublimation pump is preferably used. An evacuation means may bea turbo pump provided with a cold trap. In the film-formation chamberwhich is evacuated with the cryopump, hydrogen atoms, a compoundincluding a hydrogen atom such as H₂O, a compound including a carbonatom, and the like are exhausted. Accordingly, the concentration ofimpurities included in the oxide semiconductor film formed in thisfilm-formation chamber can be reduced.

As an example of film formation conditions, the following condition isemployed: the distance between the substrate and the target is 100 mm,the pressure is 0.6 Pa, the direct current (DC) power supply is 0.5 kW,and the atmosphere is an oxygen atmosphere (the proportion of the oxygenflow is 100%). A pulse direct current (DC) power supply is preferablebecause powder substances (also referred to as particles or dust)generated in the film formation can be reduced and the film thicknesscan be made uniform. The oxide semiconductor film preferably has athickness of 5 nm to 30 nm inclusive. Note that an appropriate thicknessof the oxide semiconductor film varies depending on the material;therefore, the thickness may be determined as appropriate depending onthe material.

Then, the insulating layer 173 having defects is formed over the gateinsulating layer 152 and the oxide semiconductor layer 171. In theformation of the insulating layer 173 having defects, the oxygen-excessmixed region 179 is formed between the oxide semiconductor layer 171 andthe insulating layer 173 having defects (see FIG. 3B).

The mixed region 179 is a mixed region of materials included in theoxide semiconductor layer 171 and in the overlying insulating layer 173having defects. By providing the mixed region, an interface between theoxide semiconductor layer 171 and the insulating layer 173 havingdefects is not clearly defined; thus, diffusion of hydrogen from theoxide semiconductor layer into the insulating layer having defects isfacilitated. For example, when a silicon oxide layer is used as theinsulating layer 173 having defects, the mixed region 179 includesoxygen, silicon, and at least one of the metal elements included in theoxide semiconductor layer.

As in this embodiment, in the case where a silicon oxide is used for theinsulating layer 173 having defects and an In—Ga—Zn—O-based film is usedas the oxide semiconductor, the mixed region 179 includes oxygen,silicon, and at least one metal element selected from In, Ga, and Zn.

The mixed region 179 may have a thickness of 0.1 nm to 30 nm(preferably, 2 nm to 10 nm). The thickness of the mixed region 179 canbe controlled by the film formation conditions of the sputtering methodat the time of forming the insulating layer 173 having defects. If thepower supply is set higher and the distance between the substrate andthe target is set shorter in the sputtering method, the mixed region 179can be formed thicker. When the sputtering method is conducted withhigher power supply, water or the like adsorbed on a surface of theoxide semiconductor layer 171 can be removed.

The provision of the mixed region 179 between the oxide semiconductorlayer 171 and the insulating layer 173 having defects promotes diffusionof hydrogen atoms, a compound including a hydrogen atom such as H₂O, acompound including a carbon atom, and the like which are included in theoxide semiconductor layer 171 into the insulating layer 173 havingdefects, and facilitates movement thereof.

The mixed region 179 needs to include excess oxygen and therefore isformed using a sputtering gas which includes much oxygen, and after theformation of the mixed region 179, the amount of oxygen in thesputtering gas may be adjusted for the formation of the insulating layer173 having defects.

The insulating layer 173 having defects may be any insulating layerhaving many defects and a silicon oxynitride layer, an aluminum oxidelayer, an aluminum oxynitride layer, or the like can be used instead ofthe silicon oxide layer. Further, a silicon nitride layer, a siliconnitride oxide layer, an aluminum nitride layer, an aluminum nitrideoxide layer, or the like may be used as the insulating layer 173 havingdefects.

In this embodiment, to form an oxygen-excess mixed region and a siliconoxide layer, the substrate 100 over which layers up to the island-shapedoxide semiconductor layer 171 are formed is heated to room temperatureor a temperature lower than 100° C., a sputtering gas includinghigh-purity oxygen from which hydrogen and moisture are removed isintroduced, and a silicon target is used.

A sputtering gas used in the formation of the insulating layer 173having defects is preferably a high-purity gas in which impurities suchas hydrogen, water, hydroxyl, or hydride are reduced to such a degreethat the concentration thereof can be expressed by the unit ppm or ppb.

For example, a silicon oxide film is formed by a pulsed DC sputteringmethod under the following condition: a boron-doped silicon target whichhas a purity of 6N (the resistivity is 0.01 Ωcm) is used; the distancebetween the substrate and the target (T-S distance) is 89 mm; thepressure is 0.4 Pa, the direct-current (DC) power source is 6 kW, andthe atmosphere is oxygen (the proportion of the oxygen flow is 100%).The film thickness is 300 nm. Note that instead of a silicon target,quartz (preferably, synthetic quartz) can be used as the target forforming the silicon oxide film. As a sputtering gas, an oxygen gas or amixed gas of oxygen and argon is used.

It is preferable that the mixed region 179 and the insulating layer 173having defects be formed while moisture remaining in the process chamberis removed so that hydrogen, hydroxyl, or moisture should not beincluded in the oxide semiconductor layer 171, the mixed region 179, orthe insulating layer 173 having defects.

Note that the mixed region 179 may be formed using silicon oxynitride,aluminum oxide, aluminum oxynitride, or the like, instead of siliconoxide.

Then, heat treatment is performed at 100° C. to 400° C., in a statewhere the insulating layer 173 having defects and the oxidesemiconductor layer 171 are in contact with each other with theoxygen-excess mixed region 179 therebetween. This heat treatment candiffuse hydrogen or moisture included in the oxide semiconductor layer171 into the oxygen-excess mixed region 179 and the insulating layer 173having defects. Since the oxygen-excess mixed region 179 is providedbetween the insulating layer 173 having defects and the oxidesemiconductor layer 171, impurities such as hydrogen, hydroxyl, ormoisture included in the island-shaped oxide semiconductor layer 171 arediffused from the oxide semiconductor layer 171 into the oxygen-excessmixed region 179 or into the insulating layer 173 having defects throughthe oxygen-excess mixed region 179.

The mixed region 179, which is provided between the oxide semiconductorlayer 171 and the insulating layer 173 having defects, includes excessoxygen, and thus has many oxygen dangling bonds as defects and has highbinding energy to impurities such as hydrogen, moisture, hydroxyl, orhydride. The provision of the oxygen-excess mixed region 179 facilitatesmovement and diffusion impurities such as hydrogen, moisture, hydroxyl,or hydride included in the oxide semiconductor layer 171 into theinsulating layer 173 having defects.

In addition, when the impurities which have been removed from the oxidesemiconductor layer 171 and diffused into the insulating layer 173having defects move back toward the oxide semiconductor layer, theoxygen-excess mixed region 179 functions as a protective layer (abarrier layer) which is bound to and stabilizes the impurities so as toprevent the impurities from entering the oxide semiconductor layer.

As described above, by removing impurities such as hydrogen, moisture,hydroxyl, or hydride which cause variation from the oxide semiconductorlayer, an oxide semiconductor layer 162 with reduced impurities can beprovided. Further, the oxygen-excess mixed region 179 which functions asa barrier layer prevents the impurities which have been diffused intothe insulating layer 173 having defects from entering the oxidesemiconductor layer 162 again; thus, the impurity concentration of theoxide semiconductor layer 162 can be kept low.

Through the above process, the oxide semiconductor layer 162 in whichthe concentration of hydrogen and hydride is reduced can be formed.

As in Embodiment 2, an oxygen-excess oxide insulating layer may beprovided instead of the oxygen-excess mixed region. An oxygen-excessoxide insulating layer produces an effect similar to the effect of theoxygen-excess mixed region.

A resist mask is formed over the insulating layer 173 having defects ina third photolithography step, and selective etching is performed toform an insulating layer 166 having defects. Then, the resist mask isremoved (see FIG. 3C).

Then, a conductive film is formed over the gate insulating layer 152,the oxide semiconductor layer 162, and the insulating layer 166 havingdefects. After that, by a fourth photolithography step, a resist mask isformed, and selective etching is performed to form a source electrodelayer 165 a and a drain electrode layer 166 b. Then, the resist mask isremoved.

As a material of the source electrode layer 165 a and the drainelectrode layer 165 b, an element selected from Al, Cr, Cu, Ta, Ti, Mo,and W, an alloy containing any of these elements as a component, analloy containing any of these the elements in combination, or the likecan be given. Further, the metal conductive film may have a single-layerstructure or a stacked-layer structure of two or more layers.

Through the above process, the thin film transistor 160 including theoxide semiconductor layer 162 in which the concentration of hydrogen andhydride is reduced can be formed (see FIG. 3D).

As described above, by removing remaining moisture in the reactionatmosphere at the time of the formation of the oxide semiconductor film,the concentration of hydrogen and hydride in the oxide semiconductorfilm can be reduced. In addition, by providing the insulating layerhaving defects over the oxide semiconductor layer with the oxygen-excessmixed region therebetween, impurities such as hydrogen or moisture inthe oxide semiconductor layer are diffused into the insulating layerhaving defects, whereby the concentration of hydrogen and hydride in theoxide semiconductor layer can be reduced. Accordingly, the oxidesemiconductor layer can be stabilized.

Even if impurities move back toward the oxide semiconductor layer 162due to heat treatment in the steps after being diffused into theinsulating layer 173 having defects, the oxygen-excess mixed region 179functioning as a barrier layer prevents the impurities from entering theoxide semiconductor layer 162. Thus, the impurity concentration of theoxide semiconductor layer 162 can be kept low.

A protective insulating layer may be provided over the insulating layerhaving defects. In this embodiment, a protective insulating layer 153 isformed over the insulating layer 166 having defects, the sourceelectrode layer 165 a, and the drain electrode layer 165 b. As theprotective insulating layer 153, a silicon nitride film, a siliconnitride oxide film, an aluminum nitride film, or the like can be used.In this embodiment, the protective insulating layer 153 is formed usinga silicon nitride film (see FIG. 3E).

Note that an oxide insulating layer may be further formed over thesource electrode layer 165 a, the drain electrode layer 165 b, and theinsulating layer 166 having defects, and the protective insulating layer153 may be formed over the oxide insulating layer. Further, aplanarization insulating layer may be formed over the protectiveinsulating layer 153.

This embodiment can be implemented in appropriate combination withanother embodiment.

As described above, a highly reliable semiconductor device with stableelectric characteristics including a thin film transistor including anoxide semiconductor layer can be provided.

Embodiment 4

In this embodiment, another example of a thin film transistor which canbe applied to a semiconductor device disclosed in this specificationwill be described.

A semiconductor device and a manufacturing method thereof in thisembodiment will be described with reference to FIGS. 4A to 4C.

Although a thin film transistor 190 is described as a single-gate thinfilm transistor, a multi-gate thin film transistor including a pluralityof channel formation regions can be formed if needed.

Hereinafter, a process for manufacturing the thin film transistor 190over a substrate 140 will be described with reference to FIGS. 4A to 4C.

First, a conductive film is formed over the substrate 140 having aninsulating surface, and then the gate electrode layer 181 is formed by afirst photolithography step. In this embodiment, a tungsten film havinga thickness of 150 nm is formed by a sputtering method as the gateelectrode layer 181.

Then, a gate insulating layer 142 is formed over the gate electrodelayer 181. In this embodiment, a silicon oxynitride layer having athickness of 100 nm is formed by a plasma CVD method as the gateinsulating layer 142.

Then, a conductive film is formed over the gate insulating layer 142. Bya second photolithography step, a resist mask is formed over theconductive film, and selective etching is performed; thus, a sourceelectrode layer 195 a and a drain electrode layer 195 b are formed.Then, the resist mask is removed.

Then, an oxide semiconductor film is formed and processed into anisland-shaped oxide semiconductor layer 141 by a third photolithographystep (see FIG. 4A). In this embodiment, the oxide semiconductor film isformed by a sputtering method using an In—Ga—Zn—O-based metal oxidetarget.

The substrate is placed in a process chamber under reduced pressure, andis heated to a temperature lower than 400° C. While moisture remainingin the process chamber is removed, a sputtering gas from which hydrogenand moisture are removed is introduced to form the oxide semiconductorfilm over the substrate 140 using metal oxide as a target. In order toremove moisture remaining in the process chamber, a capture-type vacuumpump is preferably used. For example, a cryopump, an ion pump, or atitanium sublimation pump is preferably used. An evacuation means may bea turbo pump provided with a cold trap. In the film-formation chamberwhich is evacuated with the cryopump, hydrogen atoms, a compoundincluding a hydrogen atom such as H₂O, a compound including a carbonatom, and the like are exhausted. Accordingly, the concentration ofimpurities included in the oxide semiconductor film formed in thisfilm-formation chamber can be reduced.

As an example of film formation conditions, the following condition isemployed: the distance between the substrate and the target is 100 mm,the pressure is 0.6 Pa, the direct current (DC) power supply is 0.5 kW,and the atmosphere is an oxygen atmosphere (the proportion of the oxygenflow is 100%). A pulse direct current (DC) power supply is preferablebecause powder substances (also referred to as particles or dust)generated in the film formation can be reduced and the film thicknesscan be made uniform. The oxide semiconductor film preferably has athickness of 5 nm to 30 nm inclusive. Note that an appropriate thicknessof the oxide semiconductor film varies depending on the material;therefore, the thickness may be determined as appropriate depending onthe material.

Then, an insulating layer 196 having defects is formed over the gateinsulating layer 142, the oxide semiconductor layer 141, the sourceelectrode layer 195 a, and the drain electrode layer 195 b. In theformation of the insulating layer 196 having defects, the oxygen-excessmixed region 199 is formed between the oxide semiconductor layer 141 andthe insulating layer 196 having defects.

The mixed region is a mixed region of materials included in the oxidesemiconductor layer and in the overlying insulating layer havingdefects. By providing the mixed region, an interface between the oxidesemiconductor layer and the insulating layer having defects is notclearly defined; thus, diffusion of hydrogen from the oxidesemiconductor layer into the insulating layer having defects isfacilitated. For example, when a silicon oxide layer is used as theinsulating layer having defects, the mixed region includes oxygen,silicon, and at least one of the metal elements included in the oxidesemiconductor layer.

As in this embodiment, in the case where a silicon oxide is used for theinsulating layer 196 having defects and In—Ga—Zn—O-based oxide is usedas the oxide semiconductor, the mixed region 199 includes oxygen,silicon, and at least one metal element selected from In, Ga, and Zn.

The mixed region 199 may have a thickness of 0.1 nm to 30 nm(preferably, 2 nm to 10 nm). The thickness of the mixed region 199 canbe controlled by the film formation conditions of the sputtering methodat the time of forming the insulating layer 196 having defects. If thepower supply is set higher and the distance between the substrate andthe target is set shorter in the sputtering method, the mixed region 199can be formed thicker. When the sputtering method is conducted withhigher power supply, water or the like adsorbed on a surface of theoxide semiconductor layer 141 can be removed.

The provision of the mixed region 199 between the oxide semiconductorlayer 141 and the insulating layer 196 having defects promotes diffusionof hydrogen atoms, a compound including a hydrogen atom such as H₂O, acompound including a carbon atom, and the like which are included in theoxide semiconductor layer 141 into the insulating layer 196 havingdefects, and facilitates movement thereof.

The mixed region 199 needs to include excess oxygen and therefore isformed using a sputtering gas which includes much oxygen, and after theformation of the mixed region 199, the amount of oxygen in thesputtering gas may be adjusted for the formation of the insulating layer196 having defects.

The insulating layer 196 having defects may be any insulating layerhaving many defects and a silicon oxynitride layer, an aluminum oxidelayer, an aluminum oxynitride layer, or the like can be used instead ofthe silicon oxide layer. Further, a silicon nitride layer, a siliconnitride oxide layer, an aluminum nitride layer, an aluminum nitrideoxide layer, or the like may be used as the insulating layer 196 havingdefects.

In this embodiment, to form an oxygen-excess mixed region and a siliconoxide layer, the substrate 140 over which layers up to the island-shapedoxide semiconductor layer 141, the source electrode layer 195 a, and thedrain electrode layer 195 b are formed is heated to room temperature ora temperature lower than 100° C., a sputtering gas including high-purityoxygen from which hydrogen and moisture are removed is introduced, and asilicon target is used.

A sputtering gas used in the formation of the insulating layer 196having defects is preferably a high-purity gas in which impurities suchas hydrogen, water, hydroxyl, or hydride are reduced to such a degreethat the concentration thereof can be expressed by the unit ppm or ppb.

For example, a silicon oxide film is formed by a pulsed DC sputteringmethod under the following condition: a boron-doped silicon target whichhas a purity of 6N (the resistivity is 0.01 Ωcm) is used; the distancebetween the substrate and the target (T-S distance) is 89 mm; thepressure is 0.4 Pa, the direct-current (DC) power source is 6 kW, andthe atmosphere is oxygen (the proportion of the oxygen flow is 100%).The film thickness is 300 nm. Note that instead of a silicon target,quartz (preferably, synthetic quartz) can be used as the target forforming the silicon oxide film. As a sputtering gas, an oxygen gas or amixed gas of oxygen and argon is used.

It is preferable that the mixed region 199 and the insulating layer 196having defects be formed while moisture remaining in the process chamberis removed so that hydrogen, hydroxyl, or moisture should not beincluded in the oxide semiconductor layer 141, the insulating layer 173having defects, or the mixed region 179.

Note that the mixed region 199 may be formed using silicon oxynitride,aluminum oxide, aluminum oxynitride, or the like, instead of siliconoxide.

Then, the protective insulating layer 183 is formed over the insulatinglayer 196 having defects. As the protective insulating layer 183, asilicon nitride film, a silicon nitride oxide film, an aluminum nitridefilm, or the like is used. As the protective insulating layer 183, asilicon nitride film is formed by heating the substrate 140 over whichlayers up to the insulating layer 196 having defects are formed, to atemperature of 100° C. to 400° C.; introducing a sputtering gasincluding high-purity nitrogen from which hydrogen and moisture areremoved; and using a silicon target.

In this embodiment, heat treatment at 100° C. to 400° C. is performed onthe substrate 140 in the formation of the protective insulating layer183.

This heat treatment can diffuse hydrogen or moisture included in theoxide semiconductor layer 141 into the oxygen-excess mixed region 199and the insulating layer 196 having defects. Since the oxygen-excessmixed region 199 is provided between the island-shaped oxidesemiconductor layer 141 and the oxide insulating layer 196, impuritiessuch as hydrogen, hydroxyl, or moisture included in the island-shapedoxide semiconductor layer 141 are diffused from the oxide semiconductorlayer 141 into the oxygen-excess mixed region 199 or into the oxideinsulating layer 196 through the oxygen-excess mixed region 199.

The mixed region 199, which is provided between the oxide semiconductorlayer 141 and the insulating layer 196 having defects, includes excessoxygen, and thus has many oxygen dangling bonds as defects and has highbinding energy to impurities such as hydrogen, moisture, hydroxyl, orhydride. The provision of the oxygen-excess mixed region 199 facilitatesdiffusion and movement of impurities such as hydrogen, moisture,hydroxyl, or hydride included in the oxide semiconductor layer 141 intothe insulating layer 196 having defects.

In addition, when the impurities which have been removed from the oxidesemiconductor layer 141 and diffused into the insulating layer 196having defects move back toward the oxide semiconductor layer, theoxygen-excess mixed region 199 functions as a protective layer (abarrier layer) which is bound to and stabilizes the impurities so as toprevent the impurities from entering the oxide semiconductor layer.

As described above, by removing impurities such as hydrogen, moisture,hydroxyl, or hydride which cause variation from the oxide semiconductorlayer, an oxide semiconductor layer 192 with reduced impurities can beprovided. Further, the oxygen-excess mixed region 199 which functions asa barrier layer prevents the impurities which have been diffused intothe insulating layer 196 having defects from entering the oxidesemiconductor layer 192 again; thus, the impurity concentration of theoxide semiconductor layer 192 can be kept low.

Through the above process, the thin film transistor 190 including theoxide semiconductor layer 192 in which the concentration of hydrogen andhydride is reduced can be formed (see FIG. 4C).

As in Embodiment 2, an oxygen-excess oxide insulating layer may beprovided instead of the oxygen-excess mixed region. An oxygen-excessoxide insulating layer produces an effect similar to the effect of theoxygen-excess mixed region.

As described above, by removing remaining moisture in the reactionatmosphere at the time of the formation of the oxide semiconductor film,the concentration of hydrogen and hydride in the oxide semiconductorfilm can be reduced. In addition, by providing the insulating layerhaving defects over the oxide semiconductor layer with the oxygen-excessmixed region therebetween, impurities such as hydrogen or moisture inthe oxide semiconductor layer are diffused into the insulating layerhaving defects, whereby the concentration of hydrogen and hydride in theoxide semiconductor layer can be reduced. Accordingly, the oxidesemiconductor layer can be stabilized.

Even if impurities move back toward the oxide semiconductor layer 192due to heat treatment in the steps after being diffused into theinsulating layer 196 having defects, the oxygen-excess mixed region 199functioning as a barrier layer prevents the impurities from entering theoxide semiconductor layer 192. Thus, the impurity concentration of theoxide semiconductor layer 192 can be kept low.

This embodiment can be implemented in appropriate combination withanother embodiment.

As described above, a highly reliable semiconductor device with stableelectric characteristics including a thin film transistor including anoxide semiconductor layer can be provided.

Embodiment 5

In this embodiment, another example of a thin film transistor which canbe applied to a semiconductor device disclosed in this specificationwill be described. The same portion as or a portion having a functionsimilar to those in the above embodiment can be formed in a mannersimilar to that described in the above embodiment, and also the stepssimilar to those in the above embodiment can be performed in a mannersimilar to that described in the above embodiment, and repetitivedescription is omitted. In addition, detailed description of the sameportions is not repeated.

A semiconductor device and a manufacturing method thereof in thisembodiment will be described with reference to FIGS. 5A to 5E.

FIGS. 5A to 5E illustrate an example of a cross-sectional structure of asemiconductor device. A thin film transistor 310 illustrated in FIGS. 5Ato 5E has one type of a bottom-gate structure and is also referred to asan inverted staggered thin film transistor.

Although the thin film transistor 310 is described as a single-gate thinfilm transistor, a multi-gate thin film transistor including a pluralityof channel formation regions can be formed if needed.

Hereinafter, a process for manufacturing the thin film transistor 310over a substrate 300 will be described with reference to FIGS. 5A to 5E.

First, a conductive film is formed over the substrate 300 having aninsulating surface, and then a gate electrode layer 311 is formed by afirst photolithography step. Note that a resist mask may be formed by anink jetting method. The formation of the resist mask by an ink jettingmethod does not use a photomask; thus, manufacturing cost can bereduced.

Although there is no particular limitation on a substrate which can beused as the substrate 300 having an insulating surface, it is necessarythat the substrate have at least enough heat resistance to withstandheat treatment performed later. A glass substrate formed of bariumborosilicate glass, aluminoborosilicate glass or the like can be used.

As a glass substrate, if the temperature of the heat treatment to beperformed later is high, a glass substrate whose strain point is 730° C.or higher is preferably used. As a glass substrate, a glass materialsuch as aluminosilicate glass, aluminoborosilicate glass, or bariumborosilicate glass is used, for example. Note that by containing alarger amount of barium oxide (BaO) than boron oxide, a glass substratewhich is heat-resistant and more practical can be obtained. Therefore, aglass substrate containing more BaO than B₂O₃ is preferably used.

Note that a substrate formed of an insulator such as a ceramicsubstrate, a quartz substrate, or a sapphire substrate may be usedinstead of the above glass substrate. Alternatively, crystallized glassor the like may be used.

An insulating film functioning as a base film may be provided betweenthe substrate 300 and the gate electrode layer 311. The base film has afunction of preventing diffusion of impurity elements from the substrate300, and can be formed to have a single-layer or stacked-layer structureincluding one or more of a silicon nitride layer, a silicon oxide layer,a silicon nitride oxide layer, and a silicon oxynitride layer.

The gate electrode layer 311 can be formed to have a single-layer or astacked-layer structure using a metal material such as molybdenum,titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, orscandium, or an alloy material which includes any of these materials asits main component.

For example, as a two-layer structure of the gate electrode layer 311, atwo-layer structure in which a molybdenum layer is formed over analuminum layer, a two-layer structure in which a molybdenum layer isformed over a copper layer, a two-layer structure in which a titaniumnitride layer or a tantalum nitride layer is formed over a copper layer,a two-layer structure in which a molybdenum layer is formed over atitanium nitride layer, or a two-layer structure in which a tungstenlayer is formed over a tungsten nitride layer is preferable. As athree-layer structure, a stacked-layer structure in which a tungstenlayer or a tungsten nitride layer, a layer of an alloy of aluminum andsilicon or alloy of aluminum and titanium, and a titanium nitride layeror a titanium layer are stacked is preferable.

Then, a gate insulating layer 302 is formed over the gate electrodelayer 311.

The gate insulating layer 302 can be formed to have a single-layer or astacked-layer structure including a silicon oxide layer, a siliconnitride layer, a silicon oxynitride layer, a silicon nitride oxidelayer, an aluminum oxide layer, or a hafnium oxide layer by a plasma CVDmethod, a sputtering method, or the like. For example, a siliconoxynitride layer may be formed using a deposition gas containing SiH₄,oxygen, and nitrogen by a plasma CVD method. The gate insulating layer302 has a thickness of 100 nm to 500 nm inclusive. In the case of astacked-layer structure, the first gate insulating layer with athickness of 50 nm to 200 nm inclusive and the second gate insulatinglayer with a thickness of 5 nm to 300 nm inclusive are stacked in thisorder.

In this embodiment, a silicon oxynitride layer having a thickness of 100nm is formed by a plasma CVD method as the gate insulating layer 302.

Then, the oxide semiconductor film 330 having a thickness of 2 nm to 200nm inclusive is formed over the gate insulating layer 302.

Note that before the oxide semiconductor film 330 is formed by asputtering method, dust attached to a surface of the gate insulatinglayer 302 is preferably removed by reverse sputtering in which an argongas is introduced and plasma is generated. Note that instead of an argonatmosphere, nitrogen, helium, oxygen, or the like may be used.

As the oxide semiconductor film 330, an In—Ga—Zn—O-based film, anIn—Sn—Zn—O-based oxide semiconductor film, an In—Al—Zn—O-based oxidesemiconductor film, an Sn—Ga—Zn—O-based oxide semiconductor film, anAl—Ga—Zn—O-based oxide semiconductor film, an Sn—Al—Zn—O-based oxidesemiconductor film, an In—Zn—O-based oxide semiconductor film, anSn—Zn—O-based oxide semiconductor film, an Al—Zn—O-based oxidesemiconductor film, an In—O-based oxide semiconductor film, anSn—O-based oxide semiconductor film, or a Zn—O-based oxide semiconductorfilm is used. In this embodiment, the oxide semiconductor film 330 isformed by a sputtering method using an In—Ga—Zn—O-based metal oxidetarget. A cross-sectional view at this stage is shown in FIG. 5A.Further, the oxide semiconductor film 330 can be formed by a sputteringmethod in a rare gas (typically argon) atmosphere, an oxygen atmosphere,or an atmosphere of a rare gas (typically argon) and oxygen. In the caseof film formation by a sputtering method, a target including SiO₂ at 2wt % to 10 wt % inclusive may be used.

As a target for forming the oxide semiconductor film 330 by a sputteringmethod, a metal oxide target including zinc oxide as its main componentcan be used. Another example of a metal oxide target which can be usedis a metal oxide target including In, Ga, and Zn (with a compositionratio of In₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio]). As the metal oxide targetincluding In, Ga, and Zn, a target having a composition ratio ofIn₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio] or a target having a compositionratio of In₂O₃:Ga₂O₃:ZnO=1:1:4 [molar ratio] can be used. The fillingfactor of the metal oxide target is 90% to 100% inclusive, andpreferably 95% to 99.9% inclusive. With the use of a metal oxide targetwith high filling factor, the formed oxide semiconductor film has highdensity.

A sputtering gas used in the formation of the oxide semiconductor film330 is preferably a high-purity gas in which impurities such ashydrogen, water, hydroxyl, or hydride are reduced to such a degree thatthe concentration thereof can be expressed by the unit ppm or ppb.

The substrate is placed in a process chamber under reduced pressure, andthe substrate temperature is set to 100° C. to 600° C. inclusive,preferably 200° C. to 400° C. inclusive. By forming the oxidesemiconductor film while the substrate is heated, the impurityconcentration of the formed oxide semiconductor film can be reduced. Inaddition, damage by sputtering can be reduced. While moisture remainingin the process chamber is removed, a sputtering gas from which hydrogenand moisture are removed is introduced to form the oxide semiconductorfilm 330 over the substrate 300 using metal oxide as a target. In orderto remove moisture remaining in the process chamber, a capture-typevacuum pump is preferably used. For example, a cryopump, an ion pump, ora titanium sublimation pump is preferably used. An evacuation means maybe a turbo pump provided with a cold trap. In the film-formation chamberwhich is evacuated with the cryopump, hydrogen atoms and a compoundincluding a hydrogen atom such as water (H₂O) (and preferably, acompound including a carbon atom), for example, are exhausted.Accordingly, the concentration of impurities included in the oxidesemiconductor film formed in this film-formation chamber can be reduced.

As an example of film formation conditions, the following condition isemployed: the distance between the substrate and the target is 100 mm,the pressure is 0.6 Pa, the direct current (DC) power supply is 0.5 kW,and the atmosphere is an oxygen atmosphere (the proportion of the oxygenflow is 100%). A pulse direct current (DC) power supply is preferablebecause powder substances (also referred to as particles or dust)generated in the film formation can be reduced and the film thicknesscan be made uniform. The oxide semiconductor film preferably has athickness of 5 nm to 30 nm inclusive. Note that an appropriate thicknessof the oxide semiconductor film varies depending on the material;therefore, the thickness may be determined as appropriate depending onthe material.

Then, the oxide semiconductor film 330 is processed into anisland-shaped oxide semiconductor layer by a second photolithographystep. Note that a resist mask for forming the island-shaped oxidesemiconductor layer may be formed by an ink jetting method. Theformation of the resist mask by an ink jetting method does not use aphotomask; thus, manufacturing cost can be reduced.

Then, first heat treatment is performed on the oxide semiconductorlayer. By this first heat treatment, dehydration or dehydrogenation ofthe oxide semiconductor layer can be performed. The first heat treatmentis performed at a temperature of 400° C. to 750° C. inclusive,preferably, equal to or greater than 400° C. and lower than a strainpoint of the substrate. Here, the substrate is introduced into anelectric furnace which is one of heat treatment apparatuses, and heattreatment is performed on the oxide semiconductor layer in a nitrogenatmosphere at 450° C. for one hour. After that, the oxide semiconductorlayer is prevented from being exposed to air and from again includingwater or hydrogen; thus the oxide semiconductor layer 331 is obtained(see FIG. 5B).

Note that the heat treatment apparatus is not limited to an electricfurnace, and an apparatus may be provided with a device for heating anobject by heat conduction or thermal radiation from a heater such as aresistance heater. For example, a rapid thermal annealing (RTA)apparatus such as a gas rapid thermal annealing (GRTA) apparatus or alamp rapid thermal annealing (LRTA) apparatus can be used. An LRTAapparatus is an apparatus with which an object is heated by radiation oflight (an electromagnetic wave) emitted from a lamp such as a halogenlamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, ahigh-pressure sodium lamp, or a high-pressure mercury lamp. A GRTAapparatus is an apparatus for heat treatment using a high-temperaturegas. As the gas, an inert gas which does not react with the object byheat treatment, for example, nitrogen or a rare gas such as argon, isused.

For example, as the first heat treatment, GRTA in which the substrate ismoved into an inert gas heated to a temperature as high as 650° C. to700° C., heated for several minutes, and moved out of the inert gasheated to the high temperature may be performed. With GRTA,high-temperature heat treatment for a short period of time can beachieved.

Note that in the first heat treatment, it is preferable that moisture,hydrogen, or the like be not contained in nitrogen or a rare gas such ashelium, neon, or argon. Alternatively, nitrogen or a rare gas such ashelium, neon, or argon which is introduced into a heat treatmentapparatus preferably has a purity of 6N (99.9999%) or higher, morepreferably 7N (99.99999%) or higher (that is, the impurity concentrationis 1 ppm or lower, preferably 0.1 ppm or lower).

Further, depending on conditions of the first heat treatment or amaterial of the oxide semiconductor layer, the oxide semiconductor layermay be crystallized to be a microcrystalline film or a polycrystallinefilm. For example, the oxide semiconductor layer may crystallize tobecome a microcrystalline oxide semiconductor film having acrystallinity of 90% or more, or 80% or more. Further, depending onconditions of the first heat treatment or a material of the oxidesemiconductor layer, the oxide semiconductor layer may become anamorphous oxide semiconductor layer containing no crystalline component.The oxide semiconductor layer may become an oxide semiconductor film inwhich a microcrystalline portion (with a grain diameter of 1 nm to 20 nminclusive, typically 2 nm to 4 nm inclusive) is mixed into an amorphousoxide semiconductor.

The first heat treatment of the oxide semiconductor layer can beperformed on the oxide semiconductor film 330 before being processedinto the island-shaped oxide semiconductor layer. In that case, afterthe first heat treatment, the substrate is taken out of the heattreatment apparatus and subjected to the photolithography step.

The heat treatment having an effect of dehydration or dehydrogenation onthe oxide semiconductor layer may be performed at any of the followingtimings: after the oxide semiconductor layer is formed; after a sourceelectrode layer and a drain electrode layer are formed over the oxidesemiconductor layer; and after a protective insulating layer is formedover the source electrode layer and the drain electrode layer.

Further, in the case where a contact hole is formed in the gateinsulating layer 302, the formation of the contact hole may be performedbefore or after the dehydration or dehydrogenation of the oxidesemiconductor film 330.

Note that the etching of the oxide semiconductor film is not limited towet etching and may be dry etching.

The etching conditions (such as an etchant, etching time, andtemperature) are adjusted as appropriate depending on the material sothat the film can be etched into the desired shape.

Then, a conductive film is formed over the gate insulating layer 302 andthe oxide semiconductor layer 331. The conductive film may be formed bya sputtering method or a vacuum evaporation method. As a material of thesecond conductive film, an element selected from aluminum (Al), chromium(Cr), copper (Cu), tantalum (Ta), titanium (Ti), molybdenum (Mo), andtungsten (W), an alloy containing any of these elements as a component,an alloy containing any of these the elements in combination, or thelike can be given. Further, one or more materials selected frommanganese (Mn), magnesium (Mg), zirconium (Zr), beryllium (Be), andthorium (Th) may be used. Further, the conductive film may have asingle-layer structure or a stacked-layer structure of two or morelayers. For example, a single-layer structure of an aluminum filmincluding silicon, a two-layer structure in which a titanium film isstacked over an aluminum film, a three-layer structure in which atitanium film, an aluminum film, and a titanium film are stacked in thisorder can be given. Alternatively, a film, an alloy film, or a nitridefilm which contains aluminum (Al) and one or a plurality of elementsselected from titanium (Ti), tantalum (Ta), tungsten (W), molybdenum(Mo), chromium (Cr), neodymium (Nd), and scandium (Sc) may be used.

In the case where heat treatment is performed after the formation of theconductive film, the conductive film preferably has heat resistanceenough to withstand the heat treatment.

By a third photolithography step, a resist mask is formed over theconductive film, and selective etching is performed; thus, a sourceelectrode layer 315 a and a drain electrode layer 315 b are formed.Then, the resist mask is removed (see FIG. 5C).

Ultraviolet, a KrF laser light, or an ArF laser light is used for lightexposure for forming the resist mask in the third photolithography step.A channel length L of the thin film transistor to be formed laterdepends on a width of an interval between a bottom portion of the sourceelectrode layer and a bottom portion of the drain electrode layer whichare adjacent to each other over the oxide semiconductor layer 331. Notethat when light exposure is performed for the channel length L of lessthan 25 nm, extreme ultraviolet with extremely short wavelengths ofseveral nanometers to several tens of nanometers is used for lightexposure for forming the resist mask in the third photolithography step.Light exposure with extreme ultraviolet leads to a high resolution and alarge depth of field. Accordingly, the channel length L of the thin filmtransistor to be formed later can be set to 10 nm to 1000 nm inclusive.Thus, the operation speed of a circuit can be increased, and low powerconsumption can be achieved due to extremely small off-state current.

Note that each material and etching conditions are adjusted asappropriate so that the oxide semiconductor layer 331 should not removedby etching of the conductive film.

In this embodiment, a Ti film is used as the conductive film, anIn—Ga—Zn—O-based oxide semiconductor is used as the oxide semiconductorlayer 331, and an ammonia hydrogen peroxide (a mixture of ammonia,water, and hydrogen peroxide) is used as an etchant.

Note that in the third photolithography step, the oxide semiconductorlayer 331 may be partly etched in some cases to be an oxidesemiconductor layer having a groove (a depression portion). A resistmask for forming the source electrode layer 315 a and the drainelectrode layer 315 b may be formed by an ink jetting method. Theformation of the resist mask by an ink jetting method does not use aphotomask; thus, manufacturing cost can be reduced.

Further, an oxide conductive layer may be formed between the oxidesemiconductor layer 331 and the source electrode layer 315 a and thedrain electrode layer 315 b. The oxide conductive layer and the metallayer for forming the source and drain electrode layers can be formedsuccessively. The oxide conductive layer can function as a source regionand a drain region.

When the oxide conductive layer is provided as the source region or thedrain region between the oxide semiconductor layer 331 and the sourceelectrode layer 315 a or the drain electrode layer 315 b, the sourceregion and the drain region can have lower resistance and the transistorcan operate at high speed.

Further, in order to reduce the number of photomasks used in thephotolithography steps and reduce the number of photolithography steps,an etching step may be performed with the use of a multi-tone mask whichis a light-exposure mask through which light is transmitted to have aplurality of intensities. A resist mask formed using a multi-tone maskhas a plurality of thicknesses and further can be changed in shape bybeing etched, and therefore, can be used in a plurality of etching stepsto provide different patterns. Therefore, a resist mask corresponding toat least two kinds of different patterns can be formed using onemulti-tone mask. Thus, the number of light-exposure masks can be reducedand the number of corresponding photolithography steps can also bereduced, whereby simplification of the process can be realized.

Then, plasma treatment using a gas such as N₂O, N₂, or Ar is performed.By this plasma treatment, adsorbed water or the like of an exposedsurface of the oxide semiconductor layer is removed. Plasma treatmentmay be performed using a mixed gas of oxygen and argon.

Then, an oxygen-excess oxide insulating layer 319 is formed over thegate insulating layer 302, the oxide semiconductor layer 331, the sourceelectrode layer 315 a, and the drain electrode layer 315 b. In thisembodiment, the oxygen-excess oxide insulating layer 319 is formed to bein contact with the oxide semiconductor layer 331 in a region where theoxide semiconductor layer 331 does not overlap with the source electrodelayer 315 a or the drain electrode layer 315 b.

In this embodiment, to form a silicon oxide layer (SiO_(2+x), where x ispreferably equal to or greater than 0 and less than 3) as theoxygen-excess oxide insulating layer 319, the substrate 300 over whichlayers up to the source electrode layer 315 a and the drain electrodelayer 315 b are formed is heated to room temperature or a temperaturelower than 100° C., a sputtering gas including high-purity oxygen fromwhich hydrogen and moisture are removed is introduced, and a silicontarget is used. The thickness of the oxygen-excess oxide insulatinglayer 319 may be 0.1 nm to 30 nm (preferably, 2 nm to 10 nm).

A sputtering gas used in the formation of the oxygen-excess oxideinsulating layer 319 is preferably a high-purity gas in which impuritiessuch as hydrogen, water, hydroxyl, or hydride are reduced to such adegree that the concentration thereof can be expressed by the unit ppmor ppb.

For example, a silicon oxide layer is formed by a pulsed DC sputteringmethod under the following condition: a boron-doped silicon target whichhas a purity of 6N (the resistivity is 0.01 Ωcm) is used; the distancebetween the substrate and the target (T-S distance) is 89 mm; thepressure is 0.4 Pa, the direct-current (DC) power source is 6 kW, andthe atmosphere is oxygen (the proportion of the oxygen flow is 100%).Note that instead of a silicon target, quartz (preferably, syntheticquartz) can be used as the target for forming the silicon oxide layer.As a sputtering gas, an oxygen gas or a mixed gas of oxygen and argon isused.

Note that instead of a silicon oxide layer, a silicon oxynitride layer,an aluminum oxide layer, an aluminum oxynitride layer, or the like canbe used as the oxygen-excess oxide insulating layer 319.

Then an insulating layer 316 having defects is formed over theoxygen-excess oxide insulating layer 319 without exposure to air. Theoxygen-excess oxide insulating layer 319 and the insulating layer 316having defects may be formed in the same process chamber using the sametarget.

In this embodiment, a 200-nm-thick silicon oxide layer is formed as theinsulating layer 316 having defects by a sputtering method. Thesubstrate temperature in film formation may be room temperature to 300°C. inclusive. In this embodiment, the substrate temperature is 100° C.Formation of a silicon oxide film by a sputtering method can beperformed in a rare gas (typically, argon) atmosphere, an oxygenatmosphere, or an atmosphere of a rare gas (typically, argon) andoxygen. As a target, a silicon oxide target or a silicon target can beused. For example, a silicon oxide layer can be formed by a sputteringmethod in an atmosphere of oxygen and nitrogen using a silicon target.

The insulating layer 316 having defects may be any insulating layerhaving many defects, but is preferably an inorganic insulating filmwhich does not include impurities such as moisture, hydrogen ions, or OHand which prevents the entry of them from the outside. Instead of thesilicon oxide layer, a silicon oxynitride layer, an aluminum oxidelayer, an aluminum oxynitride layer, or the like can be typically used.Further, a silicon nitride layer, a silicon nitride oxide layer, analuminum nitride layer, an aluminum nitride oxide layer, or the like maybe used as the insulating layer 316 having defects.

It is preferable that the insulating layer 316 having defects be formedwhile moisture remaining in the process chamber is removed so thathydrogen, hydroxyl, or moisture should not be included in the oxidesemiconductor layer 331 or the insulating layer 316 having defects.

In order to remove moisture remaining in the process chamber, acapture-type vacuum pump is preferably used. For example, a cryopump, anion pump, or a titanium sublimation pump is preferably used. Anevacuation means may be a turbo pump provided with a cold trap. In thefilm-formation chamber which is evacuated with the cryopump, hydrogenatoms and a compound including a hydrogen atom such as water (H₂O), forexample, are exhausted. Accordingly, the concentration of impuritiesincluded in the insulating layer 316 having defects formed in thisfilm-formation chamber can be reduced.

A sputtering gas used in the formation of the insulating layer 316having defects is preferably a high-purity gas in which impurities suchas hydrogen, water, hydroxyl, or hydride are reduced to such a degreethat the concentration thereof can be expressed by the unit ppm or ppb.

Then, second heat treatment (preferably, at 200° C. to 400° C.inclusive, for example, at 250° C. to 350° C. inclusive) is performed inan inert gas atmosphere or in an oxygen gas atmosphere. For example, thesecond heat treatment is performed at 250° C. for one hour in a nitrogenatmosphere. In the second heat treatment, a portion of the oxidesemiconductor layer (a channel formation region) is heated while beingin contact with the oxide insulating layer 319.

This heat treatment can diffuse hydrogen or moisture included in theoxide semiconductor layer 331 into the oxygen-excess oxide insulatinglayer 319 and the insulating layer 316 having defects. Since theoxygen-excess oxide insulating layer 319 is provided between the oxidesemiconductor layer 331 and the insulating layer 316 having defects,impurities such as hydrogen, hydroxyl, or moisture included in theisland-shaped oxide semiconductor layer 331 are diffused from the oxidesemiconductor layer 331 into the oxygen-excess oxide insulating layer319 or into the insulating layer 316 having defects through theoxygen-excess oxide insulating layer 319.

The oxide insulating layer 319, which is provided between the oxidesemiconductor layer 331 and the insulating layer 316 having defects,includes excess oxygen, and thus has many oxygen dangling bonds asdefects and has high binding energy to impurities such as hydrogen,moisture, hydroxyl, or hydride. The provision of the oxygen-excess oxideinsulating layer 319 facilitates diffusion and movement of impuritiessuch as hydrogen, moisture, hydroxyl, or hydride included in the oxidesemiconductor layer 331 into the insulating layer 316 having defects.

In addition, when the impurities which have been removed from the oxidesemiconductor layer 331 and diffused into the insulating layer 316having defects move back toward the oxide semiconductor layer, theoxygen-excess oxide insulating layer 319 functions as a protective layer(a barrier layer) which is bound to and stabilizes the impurities so asto prevent the impurities from entering the oxide semiconductor layer.

As described above, by removing impurities such as hydrogen, moisture,hydroxyl, or hydride which cause variation from the oxide semiconductorlayer, an oxide semiconductor layer 312 with reduced impurities can beprovided. Further, the oxygen-excess oxide insulating layer 319 whichfunctions as a barrier layer prevents the impurities which have beendiffused into the insulating layer 316 having defects from entering theoxide semiconductor layer again; thus, the impurity concentration of theoxide semiconductor layer 312 can be kept low.

Note that the heat treatment for diffusion of impurities such ashydrogen from the oxide semiconductor layer into the insulating layerhaving defects is not necessarily combined with the second heattreatment and may be performed separately.

In the above steps, the heat treatment for dehydration ordehydrogenation is performed on the formed oxide semiconductor film,whereby the oxide semiconductor film is brought into an oxygen-deficientstate and reduced in resistance that is, becomes an n-type layer, andthen the oxide insulating layer is formed in contact with the oxidesemiconductor layer, which brings part of the oxide semiconductor layerinto an oxygen-excess state. As a result, the channel formation region313 which overlaps with the gate electrode layer 311 becomes an i-typeregion. At that time, a high-resistance source region 314 a which has acarrier concentration higher than at least the channel formation region313 and overlaps with the source electrode layer 315 a, and ahigh-resistance drain region 314 b which has a carrier concentrationhigher than at least the channel formation region 313 and overlaps withthe drain electrode layer 315 b are formed in a self-aligned manner.Through the above steps, the thin film transistor 310 is formed (seeFIG. 5D).

Although an example in which an oxygen-excess oxide insulating layer isformed is described in this embodiment, an oxygen-excess mixed regionmay be provided instead of the oxygen-excess oxide insulating layer asin Embodiment 1. An oxygen-excess mixed region produces an effectsimilar to the effect of the oxygen-excess oxide insulating layer.

Heat treatment may be further performed at 100° C. to 200° C. inclusivein air for 1 hour to 30 hours inclusive. In this embodiment, the heattreatment is performed at 150° C. for 10 hours. This heat treatment maybe performed at a fixed heating temperature. Alternatively, thefollowing change in the heating temperature may be conducted pluraltimes repeatedly: the heating temperature is increased from roomtemperature to a temperature of 100° C. to 200° C. inclusive and thendecreased to room temperature. Further, this heat treatment may beperformed under reduced pressure before the formation of the insulatinglayer having defects. Under reduced pressure, the heat treatment timecan be shortened. With this heat treatment, hydrogen is introduced fromthe oxide semiconductor layer to the insulating layer having defects;thus, a normally-off thin film transistor can be obtained. Accordingly,the reliability of the semiconductor device can be improved.

By forming the high-resistance drain region 314 b (or thehigh-resistance source region 314 a) in part of the oxide semiconductorlayer which overlaps with the drain electrode layer 315 b (or the sourceelectrode layer 315 a), the reliability of the thin film transistor canbe improved. Specifically, by forming the high-resistance drain region314 b, the conductivity can vary stepwise from the drain electrode layer315 b to the high-resistance drain region 314 b and the channelformation region 313. Therefore, in the case where the thin filmtransistor operates with the drain electrode layer 315 b connected to awiring for supplying a high power supply potential VDD, thehigh-resistance drain region functions as a buffer and a high electricfield is not applied locally even if a high electric field is appliedbetween the gate electrode layer 311 and the drain electrode layer 315b; thus, the breakdown voltage of the transistor can be improved.

Further, the high-resistance source region or the high-resistance drainregion in the oxide semiconductor layer is formed in the entirethickness direction in the case where the thickness of the oxidesemiconductor layer is equal to or less than 15 nm. In the case wherethe thickness of the oxide semiconductor layer is as thick as 30 nm to50 nm inclusive, in part of the oxide semiconductor layer, that is, in aregion in the oxide semiconductor layer which is in contact with thesource electrode layer or the drain electrode layer and the vicinitythereof, resistance is reduced and a high-resistance source region or ahigh-resistance drain region is formed, while a region in the oxidesemiconductor layer which is close to the gate insulating film can bemade to be an i-type region.

A protective insulating layer may be additionally provided over theinsulating layer 316 having defects. For example, a silicon nitride filmis formed by an RF sputtering method. Since an RF sputtering method hashigh productivity, it is preferably used as a film formation method ofthe protective insulating layer. The protective insulating layer isformed using an inorganic insulating film which does not includeimpurities such as moisture, a hydrogen ion, and OH and which preventsthe entry of them from the outside; for example, a silicon nitride film,an aluminum nitride film, a silicon nitride oxide film, an aluminumnitride oxide film, or the like is used. In this embodiment, theprotective insulating layer 303 is formed as the protective insulatinglayer using a silicon nitride film (see FIG. 5E).

In this embodiment, as the protective insulating layer 303, a siliconnitride film is formed by heating the substrate 300 over which layers upto the insulating layer 316 having defects are formed, to a temperatureof 100° C. to 400° C.; introducing a sputtering gas includinghigh-purity nitrogen from which hydrogen and moisture are removed; andusing a silicon target. In this step also, it is preferable that theprotective insulating layer 303 be formed while moisture remaining inthe process chamber is removed as in the case of the insulating layer316 having defects.

A planarization insulating layer for planarization may be provided overthe protective insulating layer 303.

As described above, by removing remaining moisture in the reactionatmosphere at the time of the formation of the oxide semiconductor film,the concentration of hydrogen and hydride in the oxide semiconductorfilm can be reduced. In addition, by providing the insulating layerhaving defects over the oxide semiconductor layer with the oxygen-excessmixed region therebetween, impurities such as hydrogen or moisture inthe oxide semiconductor layer are diffused into the insulating layerhaving defects, whereby the concentration of hydrogen and hydride in theoxide semiconductor layer can be reduced. Accordingly, the oxidesemiconductor layer can be stabilized.

Even if impurities move back toward the oxide semiconductor layer 312due to heat treatment in the steps after being diffused into theinsulating layer 316 having defects, the oxygen-excess oxide insulatinglayer 319 functioning as a barrier layer prevents the impurities fromentering the oxide semiconductor layer 312. Thus, the impurityconcentration of the oxide semiconductor layer 312 can be kept low.

This embodiment can be implemented in appropriate combination withanother embodiment.

As described above, a highly reliable semiconductor device with stableelectric characteristics including a thin film transistor including anoxide semiconductor layer can be provided.

Embodiment 6

In this embodiment, another example of a thin film transistor which canbe applied to a semiconductor device disclosed in this specificationwill be described. The same portion as or a portion having a functionsimilar to those in the above embodiment can be formed in a mannersimilar to that described in the above embodiment, and also the stepssimilar to those in the above embodiment can be performed in a mannersimilar to that described in the above embodiment, and repetitivedescription is omitted. In addition, detailed description of the sameportions is not repeated.

A semiconductor device and a manufacturing method thereof in thisembodiment will be described with reference to FIGS. 6A to 6D.

FIGS. 6A to 6D illustrate an example of a cross-sectional structure of asemiconductor device. A thin film transistor 360 illustrated in FIGS. 6Ato 6D has one type of a bottom-gate structure called achannel-protective structure (also referred to as a channel-stopstructure) and is also referred to as an inverted staggered thin filmtransistor.

Although the thin film transistor 360 is described as a single-gate thinfilm transistor, a multi-gate thin film transistor including a pluralityof channel formation regions can be formed if needed.

Hereinafter, a process for manufacturing the thin film transistor 360over a substrate 320 will be described with reference to FIGS. 6A to 6D.

First, a conductive film is formed over the substrate 320 having aninsulating surface, and then the gate electrode layer 361 is formed by afirst photolithography step. Note that a resist mask may be formed by anink jetting method. The formation of the resist mask by an ink jettingmethod does not use a photomask; thus, manufacturing cost can bereduced.

The gate electrode layer 361 can be formed to have a single-layer or astacked-layer structure using a metal material such as molybdenum,titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, orscandium, or an alloy material which includes any of these materials asits main component.

Then, a gate insulating layer 322 is formed over the gate electrodelayer 361.

In this embodiment, a silicon oxynitride layer having a thickness of 100nm is formed by a plasma CVD method as the gate insulating layer 322.

Then, an oxide semiconductor film having a thickness of 2 nm to 200 nminclusive is formed over the gate insulating layer 322 and processedinto an island-shaped oxide semiconductor layer by a secondphotolithography step. In this embodiment, the oxide semiconductor filmis formed by a sputtering method using an In—Ga—Zn—O-based metal oxidetarget.

It is preferable that oxide semiconductor film be formed while moistureremaining in the process chamber is removed so that hydrogen, hydroxyl,or moisture should not be included in the oxide semiconductor film.

In order to remove moisture remaining in the process chamber, acapture-type vacuum pump is preferably used. For example, a cryopump, anion pump, or a titanium sublimation pump is preferably used. Anevacuation means may be a turbo pump provided with a cold trap. In thefilm-formation chamber which is evacuated with the cryopump, hydrogenatoms and a compound including a hydrogen atom such as water (H₂O), forexample, are exhausted. Accordingly, the concentration of impuritiesincluded in the oxide semiconductor film formed in this film-formationchamber can be reduced.

A sputtering gas used in the formation of the oxide semiconductor filmis preferably a high-purity gas in which impurities such as hydrogen,water, hydroxyl, or hydride are reduced to such a degree that theconcentration thereof can be expressed by the unit ppm or ppb.

Then, dehydration or dehydrogenation of the oxide semiconductor layer isperformed. The temperature of first heat treatment for dehydration ordehydrogenation is 400° C. to 750° C. inclusive, preferably, equal to orgreater than 400° C. and lower than a strain point of the substrate.Here, the substrate is introduced into an electric furnace which is oneof heat treatment apparatuses, and heat treatment is performed on theoxide semiconductor layer in a nitrogen atmosphere at 450° C. for onehour. After that, the oxide semiconductor layer is prevented from beingexposed to air and from again including water or hydrogen; thus theoxide semiconductor layer 332 is obtained (see FIG. 6A).

Then, plasma treatment using a gas such as N₂O, N₂, or Ar is performed.By this plasma treatment, adsorbed water or the like of an exposedsurface of the oxide semiconductor layer is removed. Plasma treatmentmay be performed using a mixed gas of oxygen and argon.

Then, an oxygen-excess oxide insulating layer is formed over the gateinsulating layer 322 and the oxide semiconductor layer 332.

In this embodiment, a silicon oxide layer (SiO_(2+x), where x ispreferably equal to or greater than 0 and less than 3) is formed as anoxygen-excess oxide insulating layer 369 using a sputtering gasincluding high-purity oxygen from which hydrogen and moisture areremoved and a silicon target. The thickness of the oxygen-excess oxideinsulating layer may be 0.1 nm to 30 nm (preferably, 2 nm to 10 nm).

Note that instead of a silicon oxide layer, a silicon oxynitride layer,an aluminum oxide layer, an aluminum oxynitride layer, or the like canbe used as the oxygen-excess oxide insulating layer.

Then an insulating layer having defects is formed over the oxygen-excessoxide insulating layer without exposure to air. The oxygen-excess oxideinsulating layer and the insulating layer having defects may be formedin the same process chamber using the same target.

In this embodiment, a 200-nm-thick silicon oxide layer is formed as theinsulating layer having defects by a sputtering method.

It is preferable that the oxygen-excess oxide insulating layer and theinsulating layer having defects be formed while moisture remaining inthe process chamber is removed so that hydrogen, hydroxyl, or moistureshould not be included in the oxide semiconductor layer 332, theoxygen-excess oxide insulating layer, or the insulating layer 366 havingdefects.

In order to remove moisture remaining in the process chamber, acapture-type vacuum pump is preferably used. For example, a cryopump, anion pump, or a titanium sublimation pump is preferably used. Anevacuation means may be a turbo pump provided with a cold trap. In thefilm-formation chamber which is evacuated with the cryopump, hydrogenatoms and a compound including a hydrogen atom such as water (H₂O), forexample, are exhausted. Accordingly, the concentration of impuritiesincluded in the insulating layer 366 having defects formed in thisfilm-formation chamber can be reduced.

A sputtering gas used in the formation of the oxygen-excess oxideinsulating layer and the insulating layer having defects is preferably ahigh-purity gas in which impurities such as hydrogen, water, hydroxyl,or hydride are reduced to such a degree that the concentration thereofcan be expressed by the unit ppm or ppb.

Then, heat treatment is performed at 100° C. to 400° C., in a statewhere the insulating layer having defects and the oxide semiconductorlayer are in contact with each other with the oxygen-excess oxideinsulating layer therebetween. This heat treatment can diffuse hydrogenor moisture included in the oxide semiconductor layer 332 into theoxygen-excess oxide insulating layer and the insulating layer havingdefects. Since the oxygen-excess oxide insulating layer is providedbetween the insulating layer having defects and the oxide semiconductorlayer, impurities such as hydrogen, hydroxyl, or moisture included inthe island-shaped oxide semiconductor layer are diffused from the oxidesemiconductor layer into the oxygen-excess oxide insulating layer orinto the insulating layer having defects through the oxygen-excess oxideinsulating layer.

The oxide insulating layer, which is provided between the oxidesemiconductor layer and the insulating layer having defects, includesexcess oxygen, and thus has many oxygen dangling bonds as defects andhas high binding energy to impurities such as hydrogen, moisture,hydroxyl, or hydride. The provision of the oxygen-excess oxideinsulating layer facilitates diffusion and movement of impurities suchas hydrogen, moisture, hydroxyl, or hydride included in the oxidesemiconductor layer into the insulating layer having defects.

In addition, when the impurities which have been removed from the oxidesemiconductor layer and diffused into the insulating layer havingdefects move back toward the oxide semiconductor layer, theoxygen-excess oxide insulating layer functions as a protective layer (abarrier layer) which is bound to and stabilizes the impurities so as toprevent the impurities from entering the oxide semiconductor layer.

By a third photolithography step, a resist mask is formed over theoxygen-excess oxide insulating layer and the insulating layer havingdefects, and selective etching is performed; thus, the oxygen-excessoxide insulating layer 369 and the insulating layer 366 having defectsare formed. Then, the resist mask is removed.

As described above, by removing impurities such as hydrogen, moisture,hydroxyl, or hydride which cause variation from the oxide semiconductorlayer, an oxide semiconductor layer 362 with reduced impurities can beprovided. Further, the oxygen-excess oxide insulating layer 369 whichfunctions as a barrier layer prevents the impurities which have beendiffused into the insulating layer 366 having defects from entering theoxide semiconductor layer again; thus, the impurity concentration of theoxide semiconductor layer 362 can be kept low.

Although an example in which an oxygen-excess oxide insulating layer isformed is described in this embodiment, an oxygen-excess mixed regionmay be provided instead of the oxygen-excess oxide insulating layer asin Embodiment 1 or 3. The oxygen-excess mixed region produces an effectsimilar to the effect of the oxygen-excess oxide insulating layer.

Then, second heat treatment (preferably, at 200° C. to 400° C.inclusive, for example, at 250° C. to 350° C. inclusive) may beperformed in an inert gas atmosphere or in an oxygen gas atmosphere. Forexample, the second heat treatment is performed at 250° C. for one hourin a nitrogen atmosphere. In the second heat treatment, a portion of theoxide semiconductor layer (a channel formation region) is heated whilebeing in contact with the oxide insulating layer 369. Note that the heattreatment for diffusion of impurities such as hydrogen from the oxidesemiconductor layer into the insulating layer having defects may becombined with the second heat treatment.

In this embodiment, the oxide semiconductor layer over which the oxideinsulating layer 369 and the insulating layer 366 having defects areformed and which is partly exposed is further subjected to heattreatment in nitrogen or an inert gas atmosphere, or under reducedpressure. By the heat treatment in nitrogen or an inert gas atmosphere,or under reduced pressure, an exposed region of the oxide semiconductorlayer which is not covered with the oxide insulating layer 369 or theinsulating layer 366 having defects is brought into an oxygen-deficientstate and reduced in resistance, that is, the exposed region can be ann-type region. For example, the heat treatment is performed at 250° C.for one hour in a nitrogen atmosphere.

By the heat treatment on the oxide semiconductor layer 332 over whichthe oxide insulating layer 369 and the insulating layer 366 havingdefects are formed in a nitrogen atmosphere, the resistance of theexposed region of the oxide semiconductor layer is reduced; thus, anoxide semiconductor layer 362 including regions with differentresistances (indicated as a shaded region and a white region in FIG. 6B)is formed.

Then, a conductive film is formed over the gate insulating layer 322,the oxide semiconductor layer 362, the oxide insulating layer 369, andthe insulating layer 366 having defects. After that, by a fourthphotolithography step, a resist mask is formed, and selective etching isperformed to form a source electrode layer 365 a and a drain electrodelayer 365 b. Then, the resist mask is removed (see FIG. 6C).

As a material of the source electrode layer 365 a and the drainelectrode layer 365 b, an element selected from Al, Cr, Cu, Ta, Ti, Mo,and W, an alloy containing any of these elements as a component, analloy containing any of these the elements in combination, or the likecan be given. Further, the metal conductive film may have a single-layerstructure or a stacked-layer structure of two or more layers.

In the above steps, the heat treatment for dehydration ordehydrogenation is performed on the formed oxide semiconductor film,whereby the oxide semiconductor film is brought into an oxygen-deficientstate and reduced in resistance, and then the oxide insulating layer isformed in contact with the oxide semiconductor film, which selectivelybrings part of the oxide semiconductor film into an oxygen-excess state.As a result, the channel formation region 363 which overlaps with thegate electrode layer 361 becomes an i-type region. At that time, asource region 364 a which has lower resistance than the channelformation region 363 and overlaps with the source electrode layer 365 a,and a drain region 364 b which has lower resistance than the channelformation region 363 and overlaps with the drain electrode layer 365 bare formed in a self-aligned manner. Through the above steps, the thinfilm transistor 360 is formed.

Heat treatment may be further performed at 100° C. to 200° C. inclusivein air for 1 hour to 30 hours inclusive. In this embodiment, the heattreatment is performed at 150° C. for 10 hours. This heat treatment maybe performed at a fixed heating temperature. Alternatively, thefollowing change in the heating temperature may be conducted pluraltimes repeatedly: the heating temperature is increased from roomtemperature to a temperature of 100° C. to 200° C. inclusive and thendecreased to room temperature. Further, this heat treatment may beperformed under reduced pressure before the formation of the oxideinsulating film. Under reduced pressure, the heat treatment time can beshortened. With this heat treatment, hydrogen is introduced from theoxide semiconductor layer to the insulating layer having defects; thus,a normally-off thin film transistor can be obtained. Accordingly, thereliability of the semiconductor device can be improved.

By forming the high-resistance drain region 364 b (or thehigh-resistance source region 364 a) which overlaps with the drainelectrode layer 365 b (or the source electrode layer 365 a) in the oxidesemiconductor layer, the reliability of the thin film transistor can beimproved. Specifically, by forming the high-resistance drain region 364b, the conductivity can vary stepwise from the drain electrode layer tothe high-resistance drain region 364 b and the channel formation region363. Therefore, in the case where the thin film transistor operates withthe drain electrode layer 365 b connected to a wiring for supplying ahigh power supply potential VDD, the high-resistance drain regionfunctions as a buffer and a high electric field is not applied locallyeven if a high electric field is applied between the gate electrodelayer 361 and the drain electrode layer 365 b; thus, the breakdownvoltage of the transistor can be improved.

A protective insulating layer 323 is formed over the source electrodelayer 365 a, drain electrode layer 365 b, the oxide insulating layer369, and the insulating layer 366 having defects. In this embodiment,the protective insulating layer 323 is formed using a silicon nitridelayer (see FIG. 6D).

Note that an oxide insulating layer may be further formed over thesource electrode layer 365 a, the drain electrode layer 365 b, the oxideinsulating layer 369, and the insulating layer 366 having defects, andthe protective insulating layer 323 may be formed over the oxideinsulating layer.

As described above, by removing remaining moisture in the reactionatmosphere at the time of the formation of the oxide semiconductor film,the concentration of hydrogen and hydride in the oxide semiconductorfilm can be reduced. In addition, by providing the insulating layerhaving defects over the oxide semiconductor layer with the oxygen-excessmixed region therebetween, impurities such as hydrogen or moisture inthe oxide semiconductor layer are diffused into the insulating layerhaving defects, whereby the concentration of hydrogen and hydride in theoxide semiconductor layer can be reduced. Accordingly, the oxidesemiconductor layer can be stabilized.

Even if impurities move back toward the oxide semiconductor layer 362due to heat treatment in the steps after being diffused into theinsulating layer 366 having defects, the oxide insulating layer 369functioning as a barrier layer prevents the impurities from entering theoxide semiconductor layer 362. Thus, the impurity concentration of theoxide semiconductor layer 362 can be kept low.

This embodiment can be implemented in appropriate combination withanother embodiment.

As described above, a highly reliable semiconductor device with stableelectric characteristics including a thin film transistor including anoxide semiconductor layer can be provided.

Embodiment 7

In this embodiment, another example of a thin film transistor which canbe applied to a semiconductor device disclosed in this specificationwill be described. A thin film transistor 380 in this embodiment can beused as the thin film transistor 110 in Embodiment 1.

In this embodiment, FIG. 7 shows an example of a thin film transistorwhose manufacturing process is partly different from that of Embodiment5. FIG. 7 is the same as FIGS. 5A to 5E except for part of the steps.Thus, the same parts as in FIGS. 5A to 5E are denoted by the samereference numerals and detailed description on the parts is omitted.

In accordance with Embodiment 5, a gate electrode layer 381 is formedover a substrate 370, and a first gate insulating layer 372 a and asecond gate insulating layer 372 b are stacked. In this embodiment, thegate insulating layer has a two-layer structure: a nitride insulatinglayer is used as the first gate insulating layer 372 a and an oxideinsulating layer is used as the second gate insulating layer 372 b.

As the oxide insulating layer, a silicon oxide layer, a siliconoxynitride layer, an aluminum oxide layer, an aluminum oxynitride layer,a hafnium oxide layer or the like can be used. As the nitride insulatinglayer, a silicon nitride layer, a silicon nitride oxide layer, analuminum nitride layer, an aluminum nitride oxide layer, or the like canbe used.

In this embodiment, a silicon nitride layer and a silicon oxide layerare stacked over the gate electrode layer 381 in this order. A gateinsulating layer having a thickness of 150 nm is formed in such a mannerthat a silicon nitride layer (SiN_(y) (y>0)) having a thickness of 50 nmto 200 nm inclusive (in this embodiment, 50 nm) is formed by asputtering method as a first gate insulating layer 372 a, and then asilicon oxide layer (SiO_(x) (x>0)) having a thickness of 5 nm to 300 nm(in this embodiment, 100 nm) inclusive is formed as a second gateinsulating layer 372 b over the first gate insulating layer 372 a.

Then, an oxide semiconductor film is formed and processed into anisland-shaped oxide semiconductor layer by a photolithography step. Inthis embodiment, the oxide semiconductor film is formed by a sputteringmethod using an In—Ga—Zn—O-based metal oxide target.

It is preferable that oxide semiconductor film be formed while moistureremaining in the process chamber is removed so that hydrogen, hydroxyl,or moisture should not be included in the oxide semiconductor film.

In order to remove moisture remaining in the process chamber, acapture-type vacuum pump is preferably used. For example, a cryopump, anion pump, or a titanium sublimation pump is preferably used. Anevacuation means may be a turbo pump provided with a cold trap. In thefilm-formation chamber which is evacuated with the cryopump, hydrogenatoms, a compound including a hydrogen atom such as water (H₂O), and thelike are exhausted. Accordingly, the impurity concentration of the oxidesemiconductor film formed in this film-formation chamber can be reduced.

A sputtering gas used in the formation of the oxide semiconductor filmis preferably a high-purity gas in which impurities such as hydrogen,water, hydroxyl, or hydride are reduced to such a degree that theconcentration thereof can be expressed by the unit ppm or ppb.

Then, dehydration or dehydrogenation of the oxide semiconductor layer isperformed. The temperature of first heat treatment for dehydration ordehydrogenation is 400° C. to 750° C. inclusive, preferably, equal to orgreater than 425° C. and lower than a strain point of the substrate.Note that in the case of the temperature of equal to or greater than425° C., the heat treatment time may be one hour or shorter, whereas inthe case of the temperature lower than 425° C., the heat treatment timeis longer than one hour. Here, the substrate is introduced into anelectric furnace which is one of heat treatment apparatuses, and heattreatment is performed on the oxide semiconductor layer in a nitrogenatmosphere. After that, the oxide semiconductor layer is prevented frombeing exposed to air and from again including water or hydrogen. Afterthat, a high-purity oxygen gas, a high-purity N₂O gas, or an ultra-dryair (with a dew point of equal to or less than −40° C., preferably equalto or less than −60° C.) is introduced into the same furnace and coolingis performed. It is preferable that the oxygen gas and the N₂O gas donot include water, hydrogen, or the like. Alternatively, the oxygen gasor the N₂O gas which is introduced into the heat treatment apparatuspreferably has a purity of 6N (99.9999%) or higher, more preferably 7N(99.99999%) or higher (that is, the impurity concentration of the oxygengas or the N₂O gas is 1 ppm or lower, preferably 0.1 ppm or lower).

Note that the heat treatment apparatus is not limited to an electricfurnace. For example, a rapid thermal annealing (RTA) apparatus such asa gas rapid thermal annealing (GRTA) apparatus or a lamp rapid thermalannealing (LRTA) apparatus can be used. An LRTA apparatus is anapparatus with which an object is heated by radiation of light (anelectromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high-pressuresodium lamp, or a high-pressure mercury lamp. In addition, the LRTAapparatus may be provided with not only a lamp but also a device whichheats an object by heat conduction or heat radiation from a heater suchas a resistance heater. GRTA is a method of heat treatment using ahigh-temperature gas. As the gas, an inert gas which does not react withthe object by heat treatment, for example, nitrogen or a rare gas suchas argon, is used. The heat treatment may be performed at 600° C. to750° C. for several minutes by an RTA method.

Further, after the first heat treatment for dehydration ordehydrogenation, heat treatment may be performed at 200° C. to 400° C.inclusive, preferably 200° C. to 300° C. inclusive, in an atmosphere ofan oxygen gas or an N₂O gas.

The first heat treatment of the oxide semiconductor layer can beperformed on the oxide semiconductor film before being processed intothe island-shaped oxide semiconductor layer. In that case, after thefirst heat treatment, the substrate is taken out of the heat treatmentapparatus and subjected to the photolithography step.

Through the above process, the entire oxide semiconductor film is placedin an oxygen-excess state to have higher resistance, that is, to be ani-type oxide semiconductor film. Thus, an oxide semiconductor layer 382whose entire region is i-type is formed.

Then, by a photolithography step, a resist mask is formed over the oxidesemiconductor layer 382, and selective etching is performed to form asource electrode layer 385 a and a drain electrode layer 385 b.

In this embodiment, a silicon oxide layer (SiO_(2+x), where x ispreferably equal to or greater than 0 and less than 3) is formed as anoxygen-excess oxide insulating layer 389 using a sputtering gasincluding high-purity oxygen from which hydrogen and moisture areremoved and a silicon target. The thickness of the oxygen-excess oxideinsulating layer 389 may be 0.1 nm to 30 nm (preferably, 2 nm to 10 nm).

Note that instead of a silicon oxide layer, a silicon oxynitride layer,an aluminum oxide layer, an aluminum oxynitride layer, or the like canbe used as the oxygen-excess oxide insulating layer 389.

Then an insulating layer 386 having defects is formed over theoxygen-excess oxide insulating layer 389 without exposure to air. Theoxygen-excess oxide insulating layer 389 and the insulating layer 386having defects may be formed in the same process chamber using the sametarget.

In this embodiment, a 200-nm-thick silicon oxide layer is formed as theinsulating layer 386 having defects by a sputtering method.

It is preferable that the oxygen-excess oxide insulating layer 389 andthe insulating layer 386 having defects be formed while moistureremaining in the process chamber is removed so that hydrogen, hydroxyl,or moisture should not be included in the oxide semiconductor layer 382,the oxygen-excess oxide insulating layer 389, or the insulating layer386 having defects.

In order to remove moisture remaining in the process chamber, acapture-type vacuum pump is preferably used. For example, a cryopump, anion pump, or a titanium sublimation pump is preferably used. Anevacuation means may be a turbo pump provided with a cold trap. In thefilm-formation chamber which is evacuated with the cryopump, hydrogenatoms and a compound including a hydrogen atom such as water (H₂O), forexample, are exhausted. Accordingly, the concentration of impuritiesincluded in the insulating layer 386 having defects formed in thisfilm-formation chamber can be reduced.

A sputtering gas used in the formation of the oxygen-excess oxideinsulating layer 389 and the insulating layer 386 having defects ispreferably a high-purity gas in which impurities such as hydrogen,water, hydroxyl, or hydride are reduced to such a degree that theconcentration thereof can be expressed by the unit ppm or ppb.

Then, heat treatment is performed at 100° C. to 400° C., in a statewhere the insulating layer 386 having defects and the oxidesemiconductor layer are in contact with each other with theoxygen-excess oxide insulating layer 389 therebetween. This heattreatment can diffuse hydrogen or moisture included in the oxidesemiconductor layer into the oxygen-excess oxide insulating layer 389and the insulating layer 386 having defects. Since the oxygen-excessoxide insulating layer 389 is provided between the insulating layer 386having defects and the oxide semiconductor layer 382, impurities such ashydrogen, hydroxyl, or moisture included in the island-shaped oxidesemiconductor layer are diffused from the oxide semiconductor layer intothe oxygen-excess oxide insulating layer 389 or into the insulatinglayer 386 having defects through the oxygen-excess oxide insulatinglayer 389.

The oxide insulating layer 389, which is provided between the oxidesemiconductor layer and the insulating layer 386 having defects,includes excess oxygen, and thus has many oxygen dangling bonds asdefects and has high binding energy to impurities such as hydrogen,moisture, hydroxyl, or hydride. The provision of the oxygen-excess oxideinsulating layer 389 facilitates movement and diffusion of impuritiessuch as hydrogen, moisture, hydroxyl, or hydride included in the oxidesemiconductor layer into the insulating layer 386 having defects.

In addition, when the impurities which have been removed from the oxidesemiconductor layer and diffused into the insulating layer 386 havingdefects move back toward the oxide semiconductor layer, theoxygen-excess oxide insulating layer 389 functions as a protective layer(a barrier layer) which is bound to and stabilizes the impurities so asto prevent the impurities from entering the oxide semiconductor layer.

As described above, by removing impurities such as hydrogen, moisture,hydroxyl, or hydride which cause variation from the oxide semiconductorlayer, the oxide semiconductor layer 382 with reduced impurities can beprovided. Further, the oxygen-excess oxide insulating layer 389 whichfunctions as a barrier layer prevents the impurities which have beendiffused into the insulating layer 386 having defects from entering theoxide semiconductor layer again; thus, the impurity concentration of theoxide semiconductor layer 382 can be kept low.

Through the above process, the thin film transistor 380 can be formed.

Then, in order to reduce variation in electric characteristics of thethin film transistor, heat treatment (equal to or greater than 150° C.and lower than 350° C.) may be performed in an inert gas atmosphere orin a nitrogen gas atmosphere. For example, the heat treatment isperformed at 250° C. for one hour in a nitrogen atmosphere. Note thatthe heat treatment for diffusion of impurities such as hydrogen from theoxide semiconductor layer into the insulating layer having defects maybe combined with this heat treatment.

Heat treatment may be further performed at 100° C. to 200° C. inclusivein air for 1 hour to 30 hours inclusive. In this embodiment, the heattreatment is performed at 150° C. for 10 hours. This heat treatment maybe performed at a fixed heating temperature. Alternatively, thefollowing change in the heating temperature may be conducted pluraltimes repeatedly: the heating temperature is increased from roomtemperature to a temperature of 100° C. to 200° C. inclusive and thendecreased to room temperature. Further, this heat treatment may beperformed under reduced pressure before the formation of the oxideinsulating layer. Under reduced pressure, the heat treatment time can beshortened. With this heat treatment, hydrogen is introduced from theoxide semiconductor layer to the oxide insulating layer; thus, anormally-off thin film transistor can be obtained. Accordingly, thereliability of the semiconductor device can be improved.

A protective insulating layer 373 is formed over the insulating layer386 having defects. In this embodiment, a 100-nm-thick silicon nitridelayer is formed as the protective insulating layer 373 by a sputteringmethod.

The protective insulating layer 373 and the first gate insulating layer372 a which are formed using a nitride insulating layer do not includeimpurities such as moisture, hydrogen, hydride, or hydroxide and preventthe entry of them from the outside.

Therefore, in a manufacturing process after the formation of theprotective insulating layer 373, entry of impurities such as moisturefrom the outside can be prevented. Further, even after a device iscompleted as a semiconductor device such as a liquid crystal displaydevice, entry of impurities such as moisture from the outside can beprevented in the long term; therefore, the long-term reliability of thedevice can be improved.

Further, the insulating layer provided between the protective insulatinglayer 373 which is formed using a nitride insulating layer and the firstgate insulating layer 372 a may be removed so that the protectiveinsulating layer 373 and the first gate insulating layer 372 a are incontact with each other.

Accordingly, impurities such as moisture, hydrogen, hydride, andhydroxide in the oxide semiconductor layer are reduced as much aspossible and reentry of such impurities is prevented, so that theimpurity concentration of the oxide semiconductor layer can be kept low.

A planarization insulating layer for planarization may be provided overthe protective insulating layer 373.

As described above, by removing remaining moisture in the reactionatmosphere at the time of the formation of the oxide semiconductor film,the concentration of hydrogen and hydride in the oxide semiconductorfilm can be reduced. In addition, by providing the insulating layerhaving defects over the oxide semiconductor layer with the oxygen-excessmixed region therebetween, impurities such as hydrogen or moisture inthe oxide semiconductor layer are diffused into the insulating layerhaving defects, whereby the concentration of hydrogen and hydride in theoxide semiconductor layer can be reduced. Accordingly, the oxidesemiconductor layer can be stabilized.

Even if impurities move back toward the oxide semiconductor layer 382due to heat treatment in the steps after being diffused into theinsulating layer 386 having defects, the oxygen-excess oxide insulatinglayer 389 functioning as a barrier layer prevents the impurities fromentering the oxide semiconductor layer 382. Thus, the impurityconcentration of the oxide semiconductor layer 382 can be kept low.

This embodiment can be implemented in appropriate combination withanother embodiment.

As described above, a highly reliable semiconductor device with stableelectric characteristics including a thin film transistor including anoxide semiconductor layer can be provided.

Embodiment 8

In this embodiment, an example of a thin film transistor which can beapplied to a semiconductor device disclosed in this specification willbe described.

In this embodiment, an example of using a light-transmitting conductivematerial for a gate electrode layer, a source electrode layer, and adrain electrode layer will be described. Except those, the thin filmtransistor can be formed in a manner similar to that in the aboveembodiments; accordingly, repetitive description of the same componentsor components having functions similar to those of above embodiment andrepetitive description of similar process will be omitted. Further,detailed description of the same parts will also be omitted.

For example, as materials of the gate electrode layer, the sourceelectrode layer, and the drain electrode layer, a conductive materialthat transmits visible light can be used. For example, In—Sn—O-basedmetal oxide, In—Sn—Zn—O-based metal oxide, In—Al—Zn—O-based metal oxide,Sn—Ga—Zn—O-based metal oxide, Al—Ga—Zn—O-based metal oxide,Sn—Al—Zn—O-based metal oxide, In—Zn—O-based metal oxide, Sn—Zn—O-basedmetal oxide, Al—Zn—O-based metal oxide, In—O-based metal oxide,Sn—O-based metal oxide, or a Zn—O-based metal oxide can be employed. Thethickness thereof is set as appropriate in the range of from 50 nm to300 nm inclusive. As a film-formation method of the metal oxide used forthe gate electrode layer, the source electrode layer, and the drainelectrode layer, a sputtering method, a vacuum evaporation method (anelectron beam evaporation method or the like), an arc discharge ionplating method or a spray method is used. In the case of film formationby a sputtering method, a target including SiO₂ at 2 wt % to 10 wt %inclusive may be used.

Note that the unit of the percentage of components in thelight-transmitting conductive film is atomic percent, and the percentageof components is evaluated by analysis using an electron probe X-raymicroanalyzer (EPMA).

In the pixel provided with the thin film transistor, when a pixelelectrode layer, another electrode layer such as a capacitor electrodelayer, or another wiring layer such as a capacitor wiring layer isformed using a conductive film that transmits visible light, a displaydevice having high aperture ratio is realized. Needless to say, it ispreferable that a gate insulating layer, an oxide insulating layer, aprotective insulating layer, and a planarization insulating layer beeach formed using a film that transmits visible light.

In this specification, a film that transmits visible light refers to afilm having such a thickness as to have visible light transmittance of75% to 100%. In the case where the film has conductivity, the film isreferred to as a transparent conductive film. Further, a conductive filmwhich is semi-transparent to visible light may be used as metal oxidefor the gate electrode layer, the source electrode layer, the drainelectrode layer, the pixel electrode layer, another electrode layer, oranother wiring layer. The conductive film which is semi-transparent tovisible light means a film having visible light transmittance of 50% to75%.

When a thin film transistor has a light-transmitting property, theaperture ratio can be increased. For small liquid crystal display panelsof 10 inches or smaller in particular, a high aperture ratio can beachieved even when the size of pixels is decreased in order to realizehigher resolution of display images by increasing the number of gatewirings, for example. Further, by using a film having alight-transmitting property for components of a thin film transistor, ahigh aperture ratio can be achieved even when one pixel is divided intoa plurality of sub-pixels in order to realize a wide viewing angle. Thatis, a high aperture ratio can be realized even when a group of thin filmtransistors are densely arranged, and thus the display region can have asufficient area. For example, in the case where one pixel includes twoto four sub-pixels, the aperture ratio can be improved because the thinfilm transistor has a light-transmitting property. Further, when astorage capacitor is formed using the same steps and the same materialsas those of the thin film transistor, the storage capacitor can alsohave a light-transmitting property; therefore, the aperture ratio can befurther increased.

This embodiment can be implemented in appropriate combination withanother embodiment.

Embodiment 9

In this embodiment, an example of a thin film transistor which can beapplied to a semiconductor device disclosed in this specification willbe described.

In this embodiment, FIG. 18 illustrates an example in which an oxidesemiconductor layer is surrounded by nitride insulating films in a crosssection. FIG. 18 is the same as FIGS. 1A to 1E except the top surfaceshape and the position of the end portion of an oxide insulating layerand the structure of a gate insulating layer; therefore, the sameportions are denoted by the same reference numerals, and detaileddescription of the same portions is omitted.

A thin film transistor 180 in FIG. 18 is a bottom-gate thin filmtransistor and includes, over the substrate 100 having an insulatingsurface, the gate electrode layer 111, a gate insulating layer 142 aformed using a nitride insulating layer, a gate insulating layer 142 bformed using an oxide insulating layer, the oxide semiconductor layer112, the oxygen-excess mixed region 119, the source electrode layer 115a, and the drain electrode layer 115 b. In addition, an insulating layer146 having defects which covers the thin film transistor 180 andoverlaps with the oxide semiconductor layer 112 with the mixed region119 therebetween is provided. A protective insulating layer 143 formedusing a nitride insulating layer is additionally provided over theinsulating layer 146 having defects. The protective insulating layer 143is in contact with the gate insulating layer 142 a, which is a nitrideinsulating layer.

Since the oxygen-excess mixed region 119 has a high binding energy tohydrogen or moisture (a hydrogen atom or a compound including a hydrogenatom such as H₂O) and these impurities are stabilized in theoxygen-excess mixed region 119 and the insulating layer 146 havingdefects, these impurities can be diffused from the oxide semiconductorlayer into the oxygen-excess mixed region and the insulating layer 146having defects, whereby these impurities can be removed from the oxidesemiconductor layer. Further, the oxygen-excess mixed region 119functions as a barrier layer against impurities which have been diffusedinto the insulating layer 146 having defects to prevent the impuritiesfrom entering the oxide semiconductor layer 112 again; thus, theimpurity concentration of the oxide semiconductor layer 112 can be keptlow. Accordingly, the thin film transistor 180 including the oxidesemiconductor layer 112 in which impurities such as hydrogen, moisture,hydroxyl, or hydride (also referred to as a hydrogen compound) whichcause variation are reduced is a highly reliable thin film transistorwith stable electric characteristics.

In the thin film transistor 180 of this embodiment, the gate insulatinglayer has a stacked structure in which a nitride insulating layer and anoxide insulating layer are stacked over the gate electrode layer.Further, before the protective insulating layer 143 formed using anitride insulating layer is formed, the insulating layer 146 havingdefects and the gate insulating layer 142 b are selectively removed soas to expose the gate insulating layer 142 a formed using a nitrideinsulating layer.

At least the area of the top surface of the insulating layer 146 havingdefects and that of the top surface of the gate insulating layer 142 bare larger than that of the top surface of the oxide semiconductor layer112, and preferably the insulating layer 146 having defects covers thethin film transistor 180.

Further, the protective insulating layer 143 is formed using a nitrideinsulating layer to cover the top surface of the insulating layer 146having defects and the side surfaces of the insulating layer 146 havingdefects and the gate insulating layer 142 b, and to be in contact withthe gate insulating layer 142 a formed using a nitride insulating layer.

For the protective insulating layer 143 and the first gate insulatinglayer 142 a which are each formed using a nitride insulating layer, aninorganic insulating film which does not include impurities such asmoisture, a hydrogen ion, and OH and which prevents the entry of themfrom the outside is used: for example, a silicon nitride film, a siliconoxynitride film, an aluminum nitride film, or an aluminum oxynitridefilm obtained by a sputtering method or a plasma CVD method is used.

In this embodiment, as the protective insulating layer 143 formed usinga nitride insulating film, a 100-nm-thick silicon nitride layer isformed by an RF sputtering method to cover the top surface and sidesurfaces of the oxide semiconductor layer 112.

With the structure illustrated in FIG. 18, an impurity such as hydrogen,moisture, hydroxyl, or hydride in the oxide semiconductor layer isreduced due to the gate insulating layer 142 b and the insulating layer146 having defects which are provided to surround and be in contact withthe oxide semiconductor layer, and the entry of moisture from theoutside in a manufacturing process after the formation of the protectiveinsulating layer 143 can be prevented because the oxide semiconductorlayer is further surrounded by the gate insulating layer 142 a and theprotective insulating layer 143 which are each formed using a nitrideinsulating layer. Moreover, the entry of moisture from the outside canbe prevented in the long term even after the device is completed as asemiconductor device, for example, as a display device; thus, thelong-term reliability of the device can be improved.

In this embodiment, one thin film transistor is surrounded by nitrideinsulating layers; however, the embodiment of the present invention isnot limited to this structure. A plurality of thin film transistors maybe surrounded by nitride insulating layers, or a plurality of thin filmtransistors in a pixel portion may be surrounded by nitride insulatinglayers. A region where the protective insulating layer 143 and the gateinsulating layer 142 a are in contact with each other may be formed soas to surround a pixel portion of the active matrix substrate.

This embodiment can be implemented in appropriate combination withanother embodiment.

Embodiment 10

In this embodiment, an example of manufacturing an active matrixlight-emitting display device using a thin film transistor and alight-emitting element using electroluminescence, in the semiconductordevice according to any of Embodiments 1 to 9 will be described.

Light-emitting elements utilizing electroluminescence are classifiedaccording to whether the light-emitting material is an organic compoundor an inorganic compound. In general, the former is referred to as anorganic EL element, and the latter is referred to as an inorganic ELelement.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer containing a light-emitting organic compound,and current flows. Then, those carriers (i.e., electrons and holes) arerecombined, and light is emitted. Owing to such a mechanism, thislight-emitting element is referred to as a current-excitationlight-emitting element.

The inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that a descriptionis made here using an organic EL element as a light-emitting element.

FIG. 9 illustrates an example of a pixel structure as an example of asemiconductor device which can be driven by a digital time grayscalemethod.

A structure and operation of a pixel which can be driven by a digitaltime grayscale driving can be applied will be described. In thisexample, one pixel includes two n-channel transistors each of whichincludes an oxide semiconductor layer as a channel formation region.

A pixel 6400 includes a switching transistor 6401, a driver transistor6402, a light-emitting element 6404, and a capacitor 6403. A gate of theswitching transistor 6401 is connected to a scan line 6406, a firstelectrode (one of a source electrode and a drain electrode) of theswitching transistor 6401 is connected to a signal line 6405, and asecond electrode (the other of the source electrode and the drainelectrode) of the switching transistor 6401 is connected to a gate ofthe driver transistor 6402. The gate of the driver transistor 6402 isconnected to a power supply line 6407 via the capacitor 6403, a firstelectrode of the driver transistor 6402 is connected to the power supplyline 6407, and a second electrode of the driver transistor 6402 isconnected to a first electrode (pixel electrode) of the light-emittingelement 6404. A second electrode of the light-emitting element 6404corresponds to a common electrode 6408. The common electrode 6408 iselectrically connected to a common potential line provided over the samesubstrate.

Note that the second electrode (the common electrode 6408) of thelight-emitting element 6404 is set to a low power supply potential. Notethat the low power supply potential is a potential lower than a highpower supply potential which is supplied to the power supply line 6407.For example, GND or 0 V may be set as the low power supply potential. Apotential difference between the high power supply potential and the lowpower supply potential is applied to the light-emitting element 6404 sothat current flows through the light-emitting element 6404, whereby thelight-emitting element 6404 emits light. Thus, each potential is set sothat the difference between the high power supply potential and the lowpower supply potential is greater than or equal to a forward thresholdvoltage of the light-emitting element 6404.

When the gate capacitance of the driver transistor 6402 is used as asubstitute for the capacitor 6403, the capacitor 6403 can be omitted.The gate capacitance of the driver transistor 6402 may be formed betweena channel region and a gate electrode.

Here, in the case of employing a voltage-input voltage driving method, avideo signal is input to the gate of the driver transistor 6402 to makethe driver transistor 6402 completely turn on or off. That is, thedriver transistor 6402 operates in a linear region, and thus, a voltagehigher than the voltage of the power supply line 6407 is applied to thegate of the driver transistor 6402. Note that a voltage greater than orequal to the sum of the power supply line voltage and V_(th) of thedriver transistor 6402 is applied to the signal line 6405.

In the case of employing an analog grayscale method instead of thedigital time grayscale method, the same pixel structure as in FIG. 9 canbe employed by inputting signals in a different way.

In the case of employing the analog grayscale method, a voltage greaterthan or equal to voltage which is the sum of forward voltage of thelight-emitting element 6404 and V_(th) of the driver transistor 6402 isapplied to the gate of the driver transistor 6402. The forward voltageof the light-emitting element 6404 refers to a voltage to obtain adesired luminance, and is larger than at least a forward thresholdvoltage. By inputting a video signal to enable the driver transistor6402 to operate in a saturation region, current can be supplied to thelight-emitting element 6404. In order that the driver transistor 6402may operate in the saturation region, the potential of the power supplyline 6407 is set higher than a gate potential of the driver transistor6402. When an analog video signal is used, it is possible to feedcurrent to the light-emitting element 6404 in accordance with the videosignal and perform analog grayscale driving.

Note that the pixel structure is not limited to that illustrated in FIG.9. For example, the pixel in FIG. 9 can further include a switch, aresistor, a capacitor, a transistor, a logic circuit, or the like.

Next, structures of the light-emitting element will be described withreference to FIGS. 10A to 10C. Here, a cross-sectional structure of apixel will be described using an example where a driver TFT is ann-channel TFT. Driver TFTs 7011, 7021, and 7001 which are used for thesemiconductor devices illustrated in FIGS. 10A to 10C can bemanufactured in a manner similar to that of the thin film transistordescribed in any of Embodiments 1-8 and are light-transmitting thin filmtransistors each including an oxide semiconductor layer.

In order to extract light emitted from the light-emitting element, atleast one of an anode and a cathode should be transparent. There arefollowing structures of a light-emitting element which is formed overthe same substrate as a thin film transistor: a top-emission structurein which light is extracted through the surface opposite to thesubstrate, a bottom-emission structure in which light is extractedthrough the surface of the substrate, and a dual-emission structure inwhich light is extracted through the surface opposite to the substrateand the surface of the substrate. The pixel structure can be applied toa light-emitting element having any of these emission structures.

A light-emitting element having a bottom emission structure will bedescribed with reference to FIG. 10A.

A cross-sectional view of a pixel in the case where the driver TFT 7011is an n-channel TFT and light is emitted from a light-emitting element7012 to a first electrode 7013 side. In FIG. 10A, the first electrode7013 of the light-emitting element 7012 is formed over alight-transmitting conductive film 7017 which is electrically connecteda drain electrode layer to the driver TFT 7011, and an EL layer 7014 anda second electrode 7015 are stacked in that order over the firstelectrode 7013.

As the light-transmitting conductive film 7017, a light-transmittingconductive film of, for example, indium oxide containing tungsten oxide,indium zinc oxide containing tungsten oxide, indium oxide containingtitanium oxide, indium tin oxide containing titanium oxide, indium tinoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded can be used.

Any of a variety of materials can be used for the first electrode 7013of the light-emitting element. For example, in the case where the firstelectrode 7013 is used as a cathode, the first electrode 7013 ispreferably formed using, for example, a material having a low workfunction such as an alkali metal such as Li or Cs; an alkaline earthmetal such as Mg, Ca, or Sr; an alloy containing any of these metals(e.g., Mg:Ag or Al:Li); or a rare earth metal such as Yb or Er. In FIG.10A, the thickness of the first electrode 7013 is set so that light canbe transmitted (preferably, about 5 nm to 30 nm). For example, analuminum film having a thickness of 20 nm is used as the first electrode7013.

Note that the light-transmitting conductive film and the aluminum filmmay be stacked and then selectively etched to form thelight-transmitting conductive film 7017 and the first electrode 7013; inthis case, the light-transmitting conductive film 7017 and the firstelectrode 7013 can be etched using the same mask, which is preferable.

The peripheral portion of the first electrode 7013 is covered with apartition 7019. The partition 7019 is formed using an organic resin filmof polyimide, acrylic, polyamide, epoxy, or the like; an inorganicinsulating film; or organic polysiloxane. It is particularly preferablethat the partition 7019 be formed using a photosensitive resin materialto have an opening over the first electrode 7013 and a sidewall of theopening be formed as an inclined surface with a continuous curvature. Inthe case where a photosensitive resin material is used for the partition7019, a step of forming a resist mask can be omitted.

The EL layer 7014 which is formed over the first electrode 7013 and thepartition 7019 may include at least a light-emitting layer and nay beformed as a single layer or a stack of plural layers. When the EL layer7014 is formed as a stack of plural layers, the EL layer 7014 is formedby stacking an electron-injection layer, an electron-transport layer, alight-emitting layer, a hole-transport layer, and a hole-injection layerin that order over the first electrode 7013 which functions as acathode. Note that not all of these layers need to be provided.

The stacking order is not limited to the above stacking order. The firstelectrode 7013 may be used as an anode, and a hole-injection layer, ahole-transport layer, a light-emitting layer, an electron-transportlayer, and an electron-injection layer may be stacked in that order overthe first electrode 7013. However, from a power consumption standpoint,it is preferable that the first electrode 7013 be used as a cathode andan electron-injection layer, an electron-transport layer, alight-emitting layer, a hole-transport layer, and a hole-injection layerbe stacked in that order over the first electrode 7013 because voltagerise in a driver circuit portion can be suppressed and power consumptioncan be reduced.

As the second electrode 7015 formed over the EL layer 7014, variousmaterials can be employed. For example, in the case where the secondelectrode 7015 is used as an anode, the second electrode 7015 ispreferably formed using, for example, a material having a high workfunction such as ZrN, Ti, W, Ni, Pt, or Cr; or a light-transmittingconductive material such as ITO, IZO, or ZnO. Further, a blocking film7016, for example, a metal which blocks light or a metal which reflectslight is provided over the second electrode 7015. In this embodiment, anITO film is used as the second electrode 7015, and a Ti film is used asthe blocking film 7016.

The light-emitting element 7012 corresponds to a region where the ELlayer 7014 including a light-emitting layer is sandwiched between thefirst electrode 7013 and the second electrode 7015. In the elementstructure illustrated in FIG. 10A, light is emitted from thelight-emitting element 7012 to the first electrode 7013 side asindicated by an arrow.

Note that in an example illustrated in FIG. 10A, a light-transmittingconductive film is used as a gate electrode layer, and alight-transmitting this film is used for the source electrode layer andthe drain electrode layer; thus, light emitted from the light-emittingelement 7012 passes through the color filter layer 7033 and thesubstrate to be emitted outside.

The color filter layer 7033 is formed by a droplet discharge method suchas an ink jetting method, a printing method, an etching method using aphotolithography technique, or the like.

The color filter layer 7033 is covered with the overcoat layer 7034, andfurther covered with the protective insulating layer 7035. Note thatalthough the overcoat layer 7034 has a small thickness in FIG. 10A, theovercoat layer 7034 has a function to planarize roughness due to thecolor filter layer 7033.

The contact hole which is formed in the protective insulating layer7035, the insulating layer 7032, and the insulating layer 7031 andreaches the drain electrode is positioned to overlap with the partition7019.

Next, a light-emitting element having a dual emission structure will bedescribed with reference to FIG. 10B.

In FIG. 10B, a first electrode 7023 of a light-emitting element 7022 isformed over a light-transmitting conductive film 7027 which iselectrically connected a drain electrode layer to the driver TFT 7021,and an EL layer 7024 and a second electrode 7025 are stacked in thatorder over the first electrode 7023.

As the light-transmitting conductive film 7027, a light-transmittingconductive film of, for example, indium oxide containing tungsten oxide,indium zinc oxide containing tungsten oxide, indium oxide containingtitanium oxide, indium tin oxide containing titanium oxide, indium tinoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded can be used.

Any of a variety of materials can be used for the first electrode 7023.For example, in the case where the first electrode 7023 is used as acathode, the first electrode 7023 is preferably formed using, forexample, a material having a low work function such as an alkali metalsuch as Li or Cs; an alkaline earth metal such as Mg, Ca, or Sr; analloy containing any of these metals (e.g., Mg:Ag or Al:Li); or a rareearth metal such as Yb or Er. In this embodiment, the first electrode7023 is used as a cathode and the thickness of the first electrode 7023is set so that light can be transmitted (preferably, about 5 nm to 30nm). For example, an aluminum film having a thickness of 20 nm is usedas the first electrode.

Note that the light-transmitting conductive film and the aluminum filmmay be stacked and then selectively etched to form thelight-transmitting conductive film 7027 and the first electrode 7023; inthis case, the light-transmitting conductive film 7027 and the firstelectrode 7023 can be etched using the same mask, which is preferable.

The peripheral portion of the first electrode 7023 is covered with apartition 7029. The partition 7029 is formed using an organic resin filmof polyimide, acrylic, polyamide, epoxy, or the like; an inorganicinsulating film; or organic polysiloxane. It is particularly preferablethat the partition 7029 be formed using a photosensitive resin materialto have an opening over the first electrode 7023 and a sidewall of theopening be formed as an inclined surface with a continuous curvature. Inthe case where a photosensitive resin material is used for the partition7029, a step of forming a resist mask can be omitted.

The EL layer 7024 which is formed over the first electrode 7023 and thepartition 7029 may include a light-emitting layer and nay be formed as asingle layer or a stack of plural layers. When the EL layer 7024 isformed as a stack of plural layers, the EL layer 7024 is formed bystacking an electron-injection layer, an electron-transport layer, alight-emitting layer, a hole-transport layer, and a hole-injection layerin that order over the first electrode 7023 which functions as acathode. Note that not all of these layers need to be provided.

The stacking order is not limited to the above stacking order. The firstelectrode 7023 may be used as an anode, and a hole-injection layer, ahole-transport layer, a light-emitting layer, an electron-transportlayer, and an electron-injection layer may be stacked in that order overthe anode. However, from a power consumption standpoint, it ispreferable that the first electrode 7023 be used as a cathode and anelectron-injection layer, an electron-transport layer, a light-emittinglayer, a hole-transport layer, and a hole-injection layer be stacked inthat order over the cathode because power consumption can be reduced.

As the second electrode 7025 formed over the EL layer 7024, variousmaterials can be employed. For example, in the case where the secondelectrode 7025 is used as an anode, the second electrode 7025 ispreferably formed using a material having a high work function, forexample, a light-transmitting conductive material such as ITO, IZO, orZnO. In this embodiment, the second electrode 7026 is used as an anodeand an ITO film including silicon oxide is formed.

The light-emitting element 7022 corresponds to a region where the ELlayer 7024 including a light-emitting layer is sandwiched between thefirst electrode 7023 and the second electrode 7025. In the elementstructure illustrated in FIG. 10B, light is emitted from thelight-emitting element 7022 to both the second electrode 7025 side andthe first electrode 7023 side as indicated by arrows.

Note that in an example illustrated in FIG. 10B, a light-transmittingconductive film is used as a gate electrode layer, and alight-transmitting this film is used for the source electrode layer andthe drain electrode layer; thus, light emitted from the light-emittingelement 7022 to the first electrode 7023 side passes through the colorfilter layer 7043 and the substrate to be emitted outside.

The color filter layer 7043 is formed by a droplet discharge method suchas an ink jetting method, a printing method, an etching method using aphotolithography technique, or the like.

The color filter layer 7043 is covered with the overcoat layer 7044, andfurther covered with the protective insulating layer 7045.

The contact hole which is formed in the protective insulating layer7045, the insulating layer 7042, and the insulating layer 7042 andreaches the drain electrode is positioned to overlap with the partition7029.

Note that when a light-emitting element having a dual emission structureis used and full color display is performed on both display surfaces,light emitted from the second electrode 7025 side does not pass throughthe color filter layer 7043; therefore, a sealing substrate providedwith another color filter layer is preferably provided over the secondelectrode 7025.

A light-emitting element having a top emission structure will bedescribed with reference to FIG. 10C.

FIG. 10C illustrates a cross-sectional view of a pixel in the case wherethe driver TFT 7001 is an n-channel TFT and light is emitted from alight-emitting element 7002 to a first second electrode 7005 side. InFIG. 10C, a drain electrode layer of the driver TFT 7001 and a firstelectrode 7003 are in contact with each other, and the driver TFT 7001and the first electrode 7003 of the light-emitting element 7002 areelectrically connected to each other. An EL layer 7004 and a secondelectrode 7005 are stacked over the first electrode 7003 in this order.

Any of a variety of materials can be used for the first electrode 7003of the light-emitting element. For example, in the case where the firstelectrode 7003 is used as a cathode, the first electrode 7003 ispreferably formed using, for example, a material having a low workfunction such as an alkali metal such as Li or Cs; an alkaline earthmetal such as Mg, Ca, or Sr; an alloy containing any of these metals(e.g., Mg:Ag or Al:Li); or a rare earth metal such as Yb or Er.

The peripheral portion of the first electrode 7003 is covered with apartition 7009. The partition 7009 is formed using an organic resin filmof polyimide, acrylic, polyamide, epoxy, or the like; an inorganicinsulating film; or organic polysiloxane. It is particularly preferablethat the partition 7009 be formed using a photosensitive resin materialto have an opening over the first electrode 7003 and a sidewall of theopening be formed as an inclined surface with a continuous curvature. Inthe case where a photosensitive resin material is used for the partition7009, a step of forming a resist mask can be omitted.

The EL layer 7004 which is formed over the first electrode 7003 and thepartition 7009 may include at least a light-emitting layer and nay beformed as a single layer or a stack of plural layers. When the EL layer7004 is formed as a stack of plural layers, the EL layer 7004 is formedby stacking an electron-injection layer, an electron-transport layer, alight-emitting layer, a hole-transport layer, and a hole-injection layerin that order over the first electrode 7003 which is used as a cathode.Note that not all of these layers need to be provided.

The stacking order is not limited to the above stacking order. Ahole-injection layer, a hole-transport layer, a light-emitting layer, anelectron-transport layer, and an electron-injection layer may be stackedin that order over the first electrode 7003 which is used as an anode.

In FIG. 10C, a hole-injection layer, a hole-transport layer, alight-emitting layer, an electron-transport layer, and anelectron-injection layer are stacked in that order over a stacked filmin which a Ti film, an aluminum film, and a Ti film are stacked in thatorder, and a stacked layer of a Mg:Ag alloy thin film and ITO is formedthereover.

In the case where the driver TFT 7001 is an n-channel TFT, it ispreferable that an electron-injection layer, an electron-transportlayer, a light-emitting layer, a hole-transport layer, and ahole-injection layer be stacked in that order over the first electrode7003 because voltage rise in a driver circuit can be suppressed andpower consumption can be reduced.

The second electrode 7005 is formed using a light-transmittingconductive material which transmits light, and for example, alight-transmitting conductive film of indium oxide containing tungstenoxide, indium zinc oxide containing tungsten oxide, indium oxidecontaining titanium oxide, indium tin oxide containing titanium oxide,indium tin oxide, indium zinc oxide, indium tin oxide to which siliconoxide is added, or the like may be used.

The light-emitting element 7002 corresponds to a region where the ELlayer 7004 including a light-emitting layer is sandwiched between thefirst electrode 7003 and the second electrode 7005. In the elementstructure illustrated in FIG. 10C, light is emitted from thelight-emitting element 7002 to the second electrode 7005 side asindicated by an arrow.

In FIG. 10C, the drain electrode layer of the driver TFT 7001 iselectrically connected to the first electrode 7003 through the contacthole formed in an insulating layer 7051 having defects, a protectiveinsulating layer 7052, a planarization insulating layer 7056, aplanarization insulating layer 7053, and an insulating layer 7055. Forthe planarization insulating layers 7036, 7046, 7053, and 7056, a resinmaterial such as polyimide, acrylic, benzocyclobutene, polyamide, orepoxy can be used. As an alternative to such resin materials, it ispossible to use a low-dielectric constant material (a low-k material), asiloxane-based resin, phosphosilicate glass (PSG), borophosphosilicateglass (BPSG), or the like. Note that the planarization insulating layers7036, 7046, 7053, and 7056 may be formed by stacking a plurality ofinsulating films formed of these materials. There is no particularlimitation on the method for forming the planarization insulating layers7036, 7046, 7053, and 7056. Depending on the material, the planarizationinsulating layer 7036, 7046, 7053, and 7056 can be formed by a methodsuch as sputtering method, an SOG method, a spin coating method, adipping method, a spray coating method, or a droplet discharge method(e.g., an ink jetting method, screen printing, or offset printing), orby using a tool (apparatus) such as a doctor knife, a roll coater, acurtain coater, a knife coater, or the like.

The partition 7009 is provided so as to insulate the first electrode7003 and a first electrode of an adjacent pixel. The partition 7009 isformed using an organic resin film of polyimide, an acrylic resin,polyamide, an epoxy resin, or the like; an inorganic insulating film; ororganic polysiloxane. It is particularly preferable that the partition7009 be formed using a photosensitive resin material to have an openingover the first electrode 7003 and a sidewall of the opening be formed asan inclined surface with a continuous curvature. In the case where aphotosensitive resin material is used for the partition 7009, a step offorming a resist mask can be omitted.

In the structure of FIG. 10C, when full color display is performed, forexample, the light-emitting element 7002 is used as a greenlight-emitting element, one of adjacent light-emitting elements is usedas a red light-emitting element, and the other is used as a bluelight-emitting element. Alternatively, a light-emitting display devicecapable of full color display may be manufactured using four kinds oflight-emitting elements, which include white light-emitting elements aswell as three kinds of light-emitting elements.

In the structure of FIG. 10C, a light-emitting display device capable offull color display may be manufactured in such a way that all of aplurality of light-emitting elements which is arranged is whitelight-emitting elements and a sealing substrate having a color filter orthe like is provided over the light-emitting element 7002. When amaterial which exhibits a single color such as white is formed and thencombined with a color filter or a color conversion layer, full colordisplay can be performed.

Any of the thin film transistors in Embodiments 1 to 9 can be used asappropriate as the driver TFTs 7001, 7011, and 7021 used for thesemiconductor devices, and the driver TFTs 7001, 7011, and 7021 can beformed using steps and materials similar to those for the thin filmtransistors in Embodiments 1 to 9. The driver TFTs 7001, 7011, and 7021include an oxygen-excess mixed region between the oxide semiconductorlayer and the insulating layer 7051, 7031, or 7041 having defects. As inEmbodiment 2, an oxygen-excess oxide insulating layer may be providedinstead of the oxygen-excess mixed region. An oxygen-excess oxideinsulating layer produces an effect similar to the effect of theoxygen-excess mixed region.

Since the oxygen-excess mixed region and insulating layer 7031, 7041,and 7051 having defects have a high binding energy to hydrogen ormoisture (a hydrogen atom or a compound including a hydrogen atom suchas H₂O) and these impurities are stabilized in the oxygen-excess mixedregion and the insulating layer having many defects, these impuritiescan be diffused from the oxide semiconductor layer into theoxygen-excess mixed region and the insulating layers 7031, 7041, and7051 having defects, whereby these impurities can be removed from theoxide semiconductor layer. Further, the oxygen-excess mixed regionfunctions as a barrier layer against impurities which have been diffusedinto the insulating layer 7031, 7041, and 7051 having defects to preventthe impurities from entering the oxide semiconductor layer again; thus,the impurity concentration of the oxide semiconductor layer can be keptlow. Accordingly, the driver TFTs 7001, 7011, and 7021 including theoxide semiconductor layer in which impurities such as hydrogen,moisture, hydroxyl, or hydride (also referred to as a hydrogen compound)which cause variation are reduced are highly reliable thin filmtransistors with stable electric characteristics.

Needless to say, display with single color light emission can also beperformed. For example, a lighting system may be formed with the use ofwhite light emission, or an area-color light-emitting device may beformed with the use of a single color light emission.

If necessary, an optical film such as a polarizing film including acircularly polarizing plate may be provided.

Although an organic EL element is described here as a light-emittingelement, an inorganic EL element can alternatively be provided as alight-emitting element.

Although the example in which a thin film transistor (a driver TFT)which controls the driving of a light-emitting element is electricallyconnected to the light-emitting element has been described, a structuremay be employed in which a TFT for controlling current is connectedbetween the driver TFT and the light-emitting element.

This embodiment can be implemented in appropriate combination withanother embodiment.

Embodiment 11

In this embodiment, an appearance and a cross section of alight-emitting display panel (also referred to as a light-emittingpanel) will be described with reference to FIGS. 11A and 11B. FIG. 11Ais a plan view of a panel in which thin film transistors and alight-emitting element which are formed over a first substrate aresealed between the first substrate and a second substrate with asealant. FIG. 11B is a cross-sectional view taken along line H-I in FIG.11A.

A sealant 4505 is provided so as to surround a pixel portion 4502,signal line driver circuits 4503 a and 4503 b, and scan line drivercircuits 4504 a and 4504 b, which are provided over a first substrate4501. In addition, a second substrate 4506 is provided over the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b. Accordingly, the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b are sealed together with afiller 4507, by the first substrate 4501, the sealant 4505, and thesecond substrate 4506. It is preferable that a panel be thus packaged(sealed) with a protective film (such as a bonding film or anultraviolet curable resin film) or a cover material with highair-tightness and little degasification so that the pixel portion 4502,the signal line driver circuits 4503 a and 4503 b, the scan line drivercircuits 4504 a and 4504 b are not exposed to air.

The pixel portion 4502, the signal line driver circuits 4503 a and 4503b, and the scan line driver circuits 4504 a and 4504 b which are formedover the first substrate 4501 each include a plurality of thin filmtransistors. A thin film transistor 4510 included in the pixel portion4502 and a thin film transistor 4509 included in the signal line drivercircuit 4503 a are illustrated as an example in FIG. 11B.

Any of the thin film transistors in Embodiments 1 to 9 can be used asappropriate as the thin film transistors 4509 and 4510, and they can beformed using steps and materials similar to those for the thin filmtransistors in Embodiments 1 to 9. The thin film transistors 4509 and4510 include an oxygen-excess mixed region (not shown) between an oxidesemiconductor layer and the insulating layer 4542 having defects. As inEmbodiment 2, an oxygen-excess oxide insulating layer may be providedinstead of the oxygen-excess mixed region. An oxygen-excess oxideinsulating layer produces an effect similar to the effect of theoxygen-excess mixed region.

Since the oxygen-excess mixed region and the insulating layer havingmany defects have a high binding energy to hydrogen or moisture (ahydrogen atom or a compound including a hydrogen atom such as H₂O) andthese impurities are stabilized in the oxygen-excess mixed region andthe insulating layer 4542 having defects, these impurities can bediffused from the oxide semiconductor layer into the oxygen-excess mixedregion and the insulating layer 4542 having defects by heat treatment,whereby these impurities can be removed from the oxide semiconductorlayer. Further, the oxygen-excess mixed region functions as a barrierlayer against impurities which have been diffused into the insulatinglayer having defects to prevent the impurities from entering the oxidesemiconductor layer again; thus, the impurity concentration of the oxidesemiconductor layer can be kept low. Accordingly, the thin filmtransistors 4509 and 4510 including the oxide semiconductor layer inwhich impurities such as hydrogen, moisture, hydroxyl, or hydride whichcause variation are reduced are highly reliable thin film transistorswith stable electric characteristics.

Note that the thin film transistor 4509 for a driver circuit has aconductive layer in a position which overlaps with a channel formationregion in the oxide semiconductor layer in the thin film transistor. Inthis embodiment, the thin film transistors 4509 and 4510 are n-channelthin film transistors.

A conductive layer 4540 is provided over part of the insulating layer4542 having defects, which overlaps with a channel formation region inan oxide semiconductor layer in the thin film transistor 4509 for thedriver circuit. The conductive layer 4540 is provided at the positionoverlapping with the channel formation region in the oxide semiconductorlayer, whereby the amount of change in threshold voltage of the thinfilm transistor 4509 before and after BT test can be reduced. Apotential of the conductive layer 4540 may be the same or different fromthat of a gate electrode layer of the thin film transistor 4509. Theconductive layer 4540 can also functions as a second gate electrodelayer. Alternatively, the potential of the conductive layer 4540 may beGND or 0 V, or the conductive layer 4540 may be in a floating state.

In addition, the conductive layer 4540 functions to block an externalelectric field, that is, to prevent an external electric field(particularly, to prevent static electricity) from effecting the inside(a circuit portion including the thin film transistor). A blockingfunction of the conductive layer 4540 can prevent variation in electriccharacteristics of the thin film transistor due to the effect ofexternal electric field such as static electricity.

Further, the insulating layer 4542 having defects which covers the oxidesemiconductor layer of the thin film transistor 4510 is formed. A sourceelectrode layer or a drain electrode layer of the thin film transistor4510 is electrically connected to a wiring layer 4550 in an openingformed in the insulating layer 4542 having defects and the insulatinglayer 4551 which are provided over the thin film transistor. The wiringlayer 4550 is formed in contact with a first electrode 4517, and thethin film transistor 4510 is electrically connected to the firstelectrode 4517 via the wiring layer 4550.

The insulating layer 4542 having defects can be formed using a materialand a method which are similar to those of the insulating layer 116having defects described in Embodiment 1.

A color filter layer 4545 is formed over the insulating layer 4551 so asto overlap with a light-emitting region of a light-emitting element4511.

Further, in order to reduce surface roughness of the color filter layer4545, the color filter layer 4545 is covered with an overcoat layer 4543functioning as a planarization insulating film.

Further, an insulating layer 4544 is formed over the overcoat layer4543. The insulating layer 4544 may be formed in a manner similar tothat of the protective insulating layer 103 described in Embodiment 1,and a silicon nitride film may be formed by a sputtering method, forexample.

Moreover, reference numeral 4511 denotes a light-emitting element. Afirst electrode 4517 which is a pixel electrode included in thelight-emitting element 4511 is electrically connected to a sourceelectrode layer or a drain electrode layer of the thin film transistor4510 via the wiring layer 4550. Note that although the light-emittingelement 4511 has a stacked-layer structure including the first electrode4517, an electroluminescent layer 4512, and a second electrode 4513 inthis embodiment, the structure of the light-emitting element 4511 is notlimited thereto. The structure of the light-emitting element 4511 can bechanged as appropriate depending on, for example, the direction in whichlight is extracted from the light-emitting element 4511.

The partition 4520 is formed using an organic resin film, an inorganicinsulating film, or organic polysiloxane. It is particularly preferablethat the partition be formed using a photosensitive material to have anopening over the first electrode 4517 and a sidewall of the opening beformed as an inclined surface with a continuous curvature.

The electroluminescent layer 4512 may be formed as a single layer or astack of plural layers.

In order to prevent entry of oxygen, hydrogen, moisture, carbon dioxide,or the like into the light-emitting element 4511, a protective film maybe formed over the second electrode 4513 and the partition 4520. As theprotective film, a silicon nitride film, a silicon nitride oxide film, aDLC film, or the like can be formed.

In addition, a variety of signals and potentials are supplied from FPCs4518 a and 4518 b to the signal line driver circuits 4503 a and 4503 b,the scan line driver circuits 4504 a and 4504 b, or the pixel portion4502.

A connection terminal electrode 4515 is formed using the same conductivefilm as the first electrode 4517 included in the light-emitting element4511. A terminal electrode 4516 is formed using the same conductive filmas a source and drain electrode layers included in the thin filmtransistors 4509.

The connection terminal electrode 4515 is electrically connected to aterminal included in the FPC 4518 a via an anisotropic conductive film4519.

The first substrate or the second substrate needs to have alight-transmitting property if it is located in the direction in whichlight is extracted from the light-emitting element 4511. In that case, alight-transmitting material such as a glass plate, a plastic plate, apolyester film, or an acrylic resin film is used.

As the filler 4507, an ultraviolet curable resin or a thermosettingresin can be used as well as an inert gas such as nitrogen or argon. Forexample, poly(vinyl chloride) (PVC), acrylic, polyimide, an epoxy resin,a silicone resin, poly(vinyl butyral) (PVB), or ethylene with vinylacetate (EVA) can be used. For example, nitrogen may be used as thefiller.

If needed, an optical film such as a polarizing plate, a circularlypolarizing plate (including an elliptically polarizing plate), or aretardation plate (a quarter-wave plate or a half-wave plate) may beprovided as appropriate on a light-emitting surface of thelight-emitting element. Further, the polarizing plate or the circularlypolarizing plate may be provided with an anti-reflection film. Forexample, anti-glare treatment by which reflected light can be diffusedby projections and depressions of the surface so as to reduce the glarecan be performed.

The sealant can be formed by a screen printing method, an ink jettingapparatus, or a dispensing apparatus. As the sealant, typically, amaterial containing a visible light curable resin, an ultravioletcurable resin, or a thermosetting resin can be used. Further, a fillermay be contained.

As the signal line driver circuits 4503 a and 4503 b and the scan linedriver circuits 4504 a and 4504 b, driver circuits formed by using asingle crystal semiconductor film or a polycrystalline semiconductorfilm over a substrate separately prepared may be mounted. Alternatively,only the signal line driver circuits or part thereof, or only the scanline driver circuits or part thereof may be separately formed and thenmounted. The structure is not limited to the structure illustrated inFIGS. 11A and 11B.

Through the above steps, a highly reliable light-emitting display device(display panel) as a semiconductor device can be manufactured.

This embodiment can be implemented in appropriate combination withanother embodiment.

Embodiment 12

The appearance and a cross section of a liquid crystal display panel,which is one embodiment of a semiconductor device, will be describedwith reference to FIGS. 8A, 8B, and 8C. FIGS. 8A and 8C are plan viewsof panels in which thin film transistors 4010 and 4011 and a liquidcrystal element 4013 are sealed between a first substrate 4001 and asecond substrate 4006 with a sealant 4005. FIG. 8B is a cross-sectionalview taken along line M-N in FIG. 8A or FIG. 8C.

The sealant 4005 is provided so as to surround a pixel portion 4002 anda scan line driver circuit 4004 which are provided over the firstsubstrate 4001. The second substrate 4006 is provided over the pixelportion 4002 and the scan line driver circuit 4004. Consequently, thepixel portion 4002 and the scan line driver circuit 4004 are sealedtogether with a liquid crystal layer 4008, by the first substrate 4001,the sealant 4005, and the second substrate 4006. A signal line drivercircuit 4003 that is formed using a single crystal semiconductor film ora polycrystalline semiconductor film over a substrate separatelyprepared is mounted in a region that is different from the regionsurrounded by the sealant 4005 over the first substrate 4001.

Note that there is no particular limitation on the connection method ofthe driver circuit which is separately formed, and a COG method, a wirebonding method, a TAB method, or the like can be used. FIG. 8Aillustrates an example in which the signal line driver circuit 4003 ismounted by a COG method. FIG. 8C illustrates an example in which thesignal line driver circuit 4003 is mounted by a TAB method.

The pixel portion 4002 and the scan line driver circuit 4004 providedover the first substrate 4001 include a plurality of thin filmtransistors. FIG. 8B illustrates the thin film transistor 4010 includedin the pixel portion 4002 and the thin film transistor 4011 included inthe scan line driver circuit 4004, as an example. The oxygen-excessoxide insulating layer 4043, an insulating layer 4041 having defects, aprotective insulating layer 4042, and an insulating layer 4021 areprovided over the thin film transistors 4010 and 4011.

Any of the thin film transistors in Embodiments 1 to 9 can be used asappropriate as the thin film transistors 4010 and 4011, and they can beformed using steps and materials similar to those for the thin filmtransistors in Embodiments 1 to 9. An oxygen-excess oxide insulatinglayer 4043 is provided between an oxide semiconductor layer and theinsulating layer having defects. As in Embodiment 1, an oxygen-excessmixed region may be provided instead of the oxygen-excess oxideinsulating layer. An oxygen-excess mixed region produces an effectsimilar to the effect of the oxygen-excess oxide insulating layer.

Since the oxygen-excess oxide insulating layer 4043 and the insulatinglayer 4041 having defects have a high binding energy to hydrogen ormoisture (a hydrogen atom or a compound including a hydrogen atom suchas H₂O) and these impurities are stabilized in the oxygen-excess oxideinsulating layer 4043 and the insulating layer 4041 having defects,these impurities can be diffused from the oxide semiconductor layer intothe oxygen-excess oxide insulating layer 4043 and the insulating layer4041 having defects, whereby these impurities can be removed from theoxide semiconductor layer. Further, the oxygen-excess oxide insulatinglayer 4043 functions as a barrier layer against impurities which havebeen diffused into the insulating layer 4041 having defects to preventthe impurities from entering the oxide semiconductor layer again; thus,the impurity concentration of the oxide semiconductor layer can be keptlow. Accordingly, the thin film transistors 4010 and 4011 including theoxide semiconductor layer in which impurities such as hydrogen,moisture, hydroxyl, or hydride which cause variation are reduced arehighly reliable thin film transistors with stable electriccharacteristics. In this embodiment, the thin film transistors 4010 and4011 are n-channel thin film transistors.

A conductive layer 4040 is provided over part of the insulating layer4021, which overlaps with a channel formation region in an oxidesemiconductor layer in the thin film transistor 4011 for the drivercircuit. The conductive layer 4040 is provided at the positionoverlapping with the channel formation region in the oxide semiconductorlayer, whereby the amount of change in threshold voltage of the thinfilm transistor 4011 before and after BT test can be reduced. Apotential of the conductive layer 4040 may be the same or different fromthat of a gate electrode layer of the thin film transistor 4011. Theconductive layer 4040 can also functions as a second gate electrodelayer. Alternatively, the potential of the conductive layer 4040 may beGND or 0 V, or the conductive layer 4044 may be in a floating state.

In addition, the conductive layer 4040 functions to block an externalelectric field, that is, to prevent an external electric field(particularly, to prevent static electricity) from effecting the inside(a circuit portion including the thin film transistor). A blockingfunction of the conductive layer 4040 can prevent variation in electriccharacteristics of the thin film transistor due to the effect ofexternal electric field such as static electricity.

A pixel electrode layer 4030 included in the liquid crystal element 4013is electrically connected to a source electrode layer or a drainelectrode layer of the thin film transistor 4010. A counter electrodelayer 4031 of the liquid crystal element 4013 is provided on the secondsubstrate 4006. A portion where the pixel electrode layer 4030, thecounter electrode layer 4031, and the liquid crystal layer 4008 overlapwith one another corresponds to the liquid crystal element 4013. Notethat the pixel electrode layer 4030 and the counter electrode layer 4031are provided with an insulating layer 4032 and an insulating layer 4033functioning as alignment films, respectively, and the liquid crystallayer 4008 is sandwiched between the pixel electrode layer 4030 and thecounter electrode layer 4031 with the insulating layers 4032 and 4033interposed therebetween.

Note that a light-transmitting substrate can be used as the firstsubstrate 4001 and the second substrate 4006; glass, ceramics, orplastics can be used. The plastic can be a fiberglass-reinforcedplastics (FRP) plate, a polyvinyl fluoride (PVF) film, a polyester film,or an acrylic resin film.

Reference numeral 4035 denotes a columnar spacer which is obtained byselective etching of an insulating film and provided in order to controlthe distance (a cell gap) between the pixel electrode layer 4030 and thecounter electrode layer 4031. Note that a spherical spacer may be used.The counter electrode layer 4031 is electrically connected to a commonpotential line formed over the substrate where the thin film transistor4010 is formed. The counter electrode layer 4031 and the commonpotential line can be electrically connected to each other viaconductive particles arranged between a pair of substrates using thecommon connection portion. Note that the conductive particles areincluded in the sealant 4005.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase while the temperature of cholestericliquid crystal is increased. Since the blue phase is generated within anonly narrow range of temperature, a liquid crystal compositioncontaining a chiral agent at 5 wt % or more is used for the liquidcrystal layer 4008 in order to improve the temperature range. The liquidcrystal composition including liquid crystal exhibiting a blue phase anda chiral agent has a short response time of 1 msec or less and isoptically isotropic; therefore, alignment treatment is not necessary andviewing angle dependence is small. In addition, since an alignment filmdoes not need to be provided and rubbing treatment is unnecessary,electrostatic breakdown caused by rubbing treatment can be prevented anddefects and damage of the liquid crystal display device can be reducedin the manufacturing process. Thus, the productivity of the liquidcrystal display device can be increased. A thin film transistorincluding an oxide semiconductor layer particularly has a possibilitythat electric characteristics of the thin film transistor maysignificantly change and deviate from the designed range by theinfluence of static electricity. Therefore, it is more effective to usea blue phase liquid crystal material for a liquid crystal display deviceincluding a thin film transistor that includes an oxide semiconductorlayer.

Note that this embodiment can also be applied to a transflective liquidcrystal display device in addition to a transmissive liquid crystaldisplay device.

Although, a polarizing plate is provided on the outer surface of thesubstrate (on the viewer side) and a coloring layer and an electrodelayer used for a display element are sequentially provided on the innersurface of the substrate in the example of the liquid crystal displaydevice, the polarizing plate may be provided on the inner surface of thesubstrate. The stacked-layer structure of the polarizing plate and thecoloring layer is not limited to the structure in this embodiment andmay be set as appropriate depending on materials of the polarizing plateand the coloring layer or conditions of the manufacturing process.Further, a light-blocking film functioning as a black matrix may beprovided in a portion other than the display portion.

The oxygen-excess oxide insulating layer 4043 and the insulating layer4041 having defects are stacked in contact with the oxide semiconductorlayer over the thin film transistors 4011 and 4010. The oxygen-excessoxide insulating layer 4043 can be formed using a material and a methodwhich are similar to those of the oxygen-excess oxide insulating layer139 described in Embodiment 2. The insulating layer 4041 having defectscan be formed using a material and a method which are similar to thoseof the insulating layer 116 having defects described in Embodiment 1.

Further, the protective insulating layer 4042 is formed on and incontact with the insulating layer 4041 having defects. The protectiveinsulating layer 4042 can be formed in a manner similar to that of theprotective insulating layer 103 described in Embodiment 1, and a siliconnitride film can be used, for example. Further, in order to reducesurface roughness of the thin film transistors, the protectiveinsulating layer 4042 is covered with the insulating layer 4021functioning as a planarization insulating film.

The insulating layer 4021 is formed as the planarizing insulating film.For the insulating layer 4021, a heat-resistant organic material such aspolyimide, acrylic, benzocyclobutene, polyamide, or epoxy can be used.As an alternative to such organic materials, it is possible to use alow-dielectric constant material (a low-k material), a siloxane-basedresin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), orthe like. Note that the insulating layer 4021 may be formed by stackinga plurality of insulating films formed of these materials.

There is no particular limitation on the method for forming theinsulating layer 4021. Depending on the material, insulating layer 4021can be formed by a method such as sputtering method, an SOG method, aspin coating method, a dipping method, a spray coating method, or adroplet discharge method (e.g., an ink jetting method, screen printing,or offset printing), or by using a tool (apparatus) such as a doctorknife, a roll coater, a curtain coater, a knife coater, or the like.When the baking step of the insulating layer 4021 and the annealing ofthe semiconductor layer are combined, a semiconductor device can bemanufactured efficiently.

The pixel electrode layer 4030 and the counter electrode layer 4031 canbe formed using a light-transmitting conductive material such as indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide (hereinafter referred to asITO), indium zinc oxide, or indium tin oxide to which silicon oxide isadded.

Conductive compositions including a conductive high molecule (alsoreferred to as a conductive polymer) can be used for the pixel electrodelayer 4030 and the counter electrode layer 4031. The pixel electrodeformed using the conductive composition preferably has a sheetresistance of 10000 Ω/square or less and a light transmittance of 70% ormore at a wavelength of 550 nm. Further, the resistivity of theconductive high molecule included in the conductive composition ispreferably 0.1 Ω·cm or less.

As the conductive high molecule, a so-called t-electron conjugatedconductive polymer can be used. Examples include polyaniline and aderivative thereof, polypyrrole and a derivative thereof, polythiopheneand a derivative thereof, and a copolymer of two or more of thesematerials.

Further, a variety of signals and potentials are supplied to the signalline driver circuit 4003 which is separately formed, the scan linedriver circuit 4004, or the pixel portion 4002 from an FPC 4018.

A connection terminal electrode 4015 is formed using the same conductivefilm as the pixel electrode layer 4030 included in the liquid crystalelement 4013. A terminal electrode 4016 is formed using the sameconductive film as a source and drain electrode layers included in thethin film transistors 4010 and 4011.

The connection terminal electrode 4015 is electrically connected to aterminal included in the FPC 4018 via an anisotropic conductive film4019.

Note that FIGS. 8A, 8B, and 8C illustrate the example in which thesignal line driver circuit 4003 is formed separately and mounted on thefirst substrate 4001; however, this embodiment is not limited to thisstructure. The scan line driver circuit may be separately formed andthen mounted, or only part of the signal line driver circuit or part ofthe scan line driver circuit may be separately formed and then mounted.

A black matrix (a light-blocking layer), an optical member (an opticalsubstrate) such as a polarizing member, a retardation member, or ananti-reflection member, and the like are provided as appropriate. Forexample, circular polarization may be obtained by using a polarizingsubstrate and a retardation substrate. In addition, a backlight, a sidelight, or the like may be used as a light source.

In an active matrix liquid crystal display device, pixel electrodesarranged in a matrix are driven to form a display pattern on a screen.Specifically, voltage is applied between a selected pixel electrode anda counter electrode corresponding to the pixel electrode, so that aliquid crystal layer provided between the pixel electrode and thecounter electrode is optically modulated and this optical modulation isrecognized as a display pattern by an observer.

In displaying moving images, a liquid crystal display device has aproblem that a long response time of liquid crystal molecules causesafterimages or blurring of moving images. In order to improve themoving-image characteristics of a liquid crystal display device, adriving method called black insertion is employed in which black isdisplayed on the whole screen every other frame period.

Further, a driving method called double-frame rate driving may beemployed in which a vertical synchronizing frequency is set 1.5 times ormore, or 2 times or more as high as a usual vertical synchronizingfrequency to improve the response speed.

Further alternatively, in order to improve the moving-imagecharacteristics of a liquid crystal display device, a driving method maybe employed in which a plurality of LEDs (light-emitting diodes) or aplurality of EL light sources are used to form a surface light source asa backlight, and each light source of the surface light source isindependently driven in a pulsed manner in one frame period. As thesurface light source, three or more kinds of LEDs may be used or an LEDemitting white light may be used. Since a plurality of LEDs can becontrolled independently, the timing at which the LED emits light can besynchronized with the timing at which the liquid crystal layer isoptically modulated. In this driving method, part of the LEDs can beturned off; therefore, an effect of reducing power consumption can beobtained particularly in the case of displaying an image having a largeblack part.

By combining these driving methods, the display characteristics of aliquid crystal display device, such as moving-image characteristics, canbe improved as compared to those of conventional liquid crystal displaydevices.

Since the thin film transistor is easily broken due to staticelectricity or the like, a protective circuit is preferably providedover the same substrate as the pixel portion and the driver circuit. Theprotective circuit is preferably formed using a non-linear elementincluding an oxide semiconductor layer. For example, a protectivecircuit is provided between the pixel portion and a scan line inputterminal and between the pixel portion and a signal line input terminal.In this embodiment, a plurality of protective circuits are provided soas to prevent breakage of the a pixel transistor and the like which canbe caused when a surge voltage due to static electricity or the like isapplied to a scan line, a signal line, and a capacitor bus line. Theprotective circuit is formed so as to release charge to a common wiringwhen a surge voltage is applied to the protective circuit. Further, theprotective circuit includes non-linear elements arranged in parallel toeach other with the scan line therebetween. The non-linear element is atwo-terminal element such as a diode or a three-terminal element such asa transistor. For example, the non-linear element can be formed in thesame steps as the thin film transistor in the pixel portion. Forexample, characteristics similar to those of a diode can be obtained byconnection of a gate terminal to a drain terminal of the non-linearelement.

For the liquid crystal display module, a twisted nematic (TN) mode, anin-plane-switching (IPS) mode, a fringe field switching (FFS) mode, anaxially symmetric aligned micro-cell (ASM) mode, an opticallycompensated birefringence (OCB) mode, a ferroelectric liquid crystal(FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, or the likecan be employed.

There is no particular limitation in the semiconductor device disclosedin this specification, and a liquid crystal display device including aTN liquid crystal, an OCB liquid crystal, an STN liquid crystal, a VAliquid crystal, an ECB liquid crystal, a GH liquid crystal, a polymerdispersed liquid crystal, a discotic liquid crystal, or the like can beused. In particular, a normally black liquid crystal panel such as atransmissive liquid crystal display device utilizing a verticalalignment (VA) mode is preferable. Some examples are given as thevertical alignment mode. For example, a multi-domain vertical alignment(MVA) mode, a patterned vertical alignment (PVA) mode, and an ASV modecan be used.

Further, this embodiment can also be applied to a VA liquid crystaldisplay device. The VA mode of a liquid crystal display device is a kindof mode in which alignment of liquid crystal molecules of a liquidcrystal display panel is controlled. In the VA liquid crystal displaydevice, liquid crystal molecules are aligned in a vertical directionwith respect to a panel surface when no voltage is applied. Further, amethod called multi-domain or multi-domain design, by which a pixel isdivided into some regions (subpixels), and liquid crystal molecules arealigned in different directions in their respective regions, can beused.

This embodiment can be implemented in appropriate combination withanother embodiment.

Embodiment 13

In this embodiment, an example of an electronic paper will be describedas a semiconductor device of an embodiment of the present invention.

FIG. 12 illustrates an active matrix electronic paper as an example of asemiconductor device to which an embodiment of the present invention isapplied. Any of the thin film transistors in Embodiments 1 to 9 can beused as appropriate as a thin film transistor 581, and the thin filmtransistor 581 can be formed using steps and materials similar to thosefor the thin film transistors in Embodiments 1 to 9. The thin filmtransistor 581 includes an oxygen-excess mixed region between an oxidesemiconductor layer and the insulating layer 583 having defects. As inEmbodiment 2, an oxygen-excess oxide insulating layer may be providedinstead of the oxygen-excess mixed region. An oxygen-excess oxideinsulating layer produces an effect similar to the effect of theoxygen-excess mixed region.

Since the oxygen-excess mixed region and the insulating layer 583 havingdefects have a high binding energy to hydrogen or moisture (a hydrogenatom or a compound including a hydrogen atom such as H₂O) and theseimpurities are stabilized in the oxygen-excess mixed region and theinsulating layer having many defects, these impurities can be diffusedfrom the oxide semiconductor layer into the oxygen-excess mixed regionand the insulating layer 583 having defects, whereby these impuritiesare removed from the oxide semiconductor layer. Further, theoxygen-excess mixed region functions as a barrier layer againstimpurities which have been diffused into the insulating layer 583 havingdefects to prevent the impurities from entering the oxide semiconductorlayer again; thus, the impurity concentration of the oxide semiconductorlayer can be kept low. Accordingly, the thin film transistor 581including an oxide semiconductor layer in which impurities such ashydrogen, moisture, hydroxyl, or hydride (also referred to as a hydrogencompound) which cause variation are reduced are highly reliable thinfilm transistors with stable electric characteristics.

The electronic paper of FIG. 12 is an example of a display device inwhich a twisting ball display system is employed. The twisting balldisplay system refers to a method in which spherical particles eachcolored in black and white are arranged between a first electrode layerand a second electrode layer which are electrode layers used in adisplay element, and a potential difference is generated between thefirst electrode layer and the second electrode layer to controlorientation of the spherical particles, so that display is performed.

The thin film transistor 581 provided over a substrate 580 is abottom-gate thin film transistor. A source electrode layer or a drainelectrode layer of the thin film transistor 581 is in contact with andelectrically connected to a first electrode layer 587 in an openingformed in the insulating layer 583 having defects, a protectiveinsulating layer 584, and in an insulating layer 585.

Between the first electrode layer 587 and a second electrode layer 588formed on a substrate 596, spherical particles each having a blackregion 590 a, a white region 590 b, and a cavity 594 filled with liquidaround the regions are provided. Space around the spherical particles isfilled with a filler 595 such as a resin (see FIG. 12). In thisembodiment, the first electrode layer 587 corresponds to a pixelelectrode, and the second electrode layer 588 on the counter substrate596 corresponds to a common electrode.

Further, instead of the twisting ball, an electrophoretic element can beused. A microcapsule having a diameter of about 10 μm to 200 μm in whichtransparent liquid, positively charged white microparticles, andnegatively charged black microparticles are encapsulated, is used. Inthe microcapsules which are provided between the first electrode layerand the second electrode layer, when an electric field is applied by thefirst electrode layer and the second electrode layer, the whitemicroparticles and the black microparticles move to opposite sides fromeach other, so that white or black can be displayed. A display elementusing this principle is an electrophoretic display element and is calledan electronic paper in general. The electrophoretic display element hashigher reflectance than a liquid crystal display element, and thus, anauxiliary light is unnecessary, power consumption is low, and a displayportion can be recognized in a dim environment. In addition, even whenpower is not supplied to the display portion, an image which has beendisplayed once can be maintained. Accordingly, a displayed image can bestored even when the semiconductor device having a display function(which may be referred to simply as a display device or a semiconductordevice provided with a display device) is distanced from a radiowavesource.

Through the above steps, a highly reliable electronic paper as asemiconductor device can be manufactured.

This embodiment can be implemented in appropriate combination withanother embodiment.

Embodiment 14

The semiconductor device disclosed in this specification can be appliedto a variety of electronic devices (including game machines). Examplesof such electronic devices are a television device (also referred to asa television or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone handset (also referred to as amobile phone or a mobile phone device), a portable game console, aportable information terminal, an audio playback device, a large-sizedgame machine such as a pinball machine, and the like.

FIG. 13A illustrates a cellular phone 1600. The cellular phone 1600includes a housing 1601 in which a display portion 1602 is incorporated,operation buttons 1603 a and 1603 b, an external connection port 1604, aspeaker and 1605, microphone 1606.

Information can be input to the cellular phone 1600 illustrated in FIG.13A by touching the display portion 1602 with a finger or the like.Further, operation such as making calls and texting can be performed bytouching the display portion 1602 with a finger or the like.

There are mainly three screen modes of the display portion 1602. Thefirst mode is a display mode mainly for displaying images. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode in which two modes of the display modeand the input mode are combined.

For example, in the case of making a call or texting, the displayportion 1602 is placed in a text input mode mainly for inputting text,and characters displayed on a screen can be input. In this case, it ispreferable to display a keyboard or number buttons on almost the entirearea of the screen of the display portion 1602.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside thecellular phone 1600, display on the screen of the display portion 1602can be automatically switched by detecting the direction of the cellularphone 1600 (whether the cellular phone 1600 is placed horizontally orvertically for a landscape mode or a portrait mode).

Further, the screen modes are switched by touching the display portion1602 or operating the operation button 1603 of the housing 1601.Alternatively, the screen modes can be switched depending on kinds ofimages displayed on the display portion 1602. For example, when a signalfor an image displayed on the display portion is data of moving images,the screen mode is switched to the display mode. When the signal is textdata, the screen mode is switched to the input mode.

Further, in the input mode, a signal is detected by an optical sensor inthe display portion 1602 and if input by touching the display portion1602 is not performed for a certain period, the screen mode may becontrolled so as to be switched from the input mode to the display mode.

The display portion 1602 can also function as an image sensor. Forexample, an image of a palm print, a fingerprint, or the like is takenby touching the display portion 1602 with the palm or the finger,whereby personal authentication can be performed. Moreover, when abacklight or sensing light source which emits near-infrared light isprovided in the display portion, an image of finger veins, palm veins,or the like can be taken.

Any of the semiconductor devices described in the above embodiments canbe applied to the display portion 1602. For example, a plurality of thinfilm transistors described in the above embodiments can be arranged asswitching elements in pixels.

FIG. 13B illustrates another example of a cellular phone. A portableinformation terminal such as the one illustrated in FIG. 13B can have aplurality of functions. For example, in addition to a telephonefunction, such a portable information terminal can have a function ofprocessing a variety of pieces of data by incorporating a computer.

The portable information terminal illustrated in FIG. 13B includes ahousing 1800 and a housing 1801. The housing 1800 includes a displaypanel 1802, a speaker 1803, a microphone 1804, a pointing device 1806, acamera lens 1807, an external connection terminal 1808, and the like.The housing 1801 includes a keyboard 1810, an external memory slot 1811,and the like. In addition, an antenna is incorporated in the housing1801.

Further, the display panel 1802 functions as a touch screen. A pluralityof operation keys 1805 which is displayed is indicated by dashed linesin FIG. 13B.

Further, in addition to the above structure, a contactless IC chip, asmall memory device, or the like may be incorporated.

Any of the semiconductor devices described in the above embodiments canbe used for the display panel 1802 and the orientation of display ischanged as appropriate depending on an application mode. Further, thecamera lens 1807 is provided in the same plane as the display portion1802; therefore, the portable information terminal can be used forvideophone calls. The speaker 1803 and the microphone 1804 can be usedfor videophone calls, recording and playing sound, and the like withoutbeing limited to voice calls. Moreover, the housings 1800 and 1801 whichare developed in FIG. 13B can be slid so that one is lapped over theother; therefore, the size of the portable information terminal can bereduced, which makes the portable information terminal suitable forbeing carried.

The external connection terminal 1808 can be connected to an AC adaptorand various types of cables such as a USB cable so that charging anddata communication with a personal computer or the like are possible.Furthermore, a large amount of data can be stored and moved with astorage medium inserted into the external memory slot 1811.

In addition to the above described functions, the portable informationterminal may have an infrared communication function, a televisionreceiver function, and the like.

FIG. 14A illustrates a television device 9600. In the television device9600, a display portion 9603 is incorporated in a housing 9601. Thedisplay portion 9603 can display images. Here, the housing 9601 issupported by a stand 9605.

The television device 9600 can be operated with an operation switch ofthe housing 9601 or a separate remote control 9610. Channels can beswitched and volume can be controlled with operation keys 9609 of theremote control 9610, whereby an image displayed on the display portion9603 can be controlled. Moreover, the remote control 9610 may beprovided with a display portion 9607 for displaying data output from theremote control 9610.

Note that the television device 9600 is provided with a receiver, amodem, and the like. With the receiver, general TV broadcasts can bereceived. Moreover, when the display device is connected to acommunication network with or without wires via the modem, one-way (froma sender to a receiver) or two-way (e.g., between a sender and areceiver or between receivers) information communication can beperformed.

Any of the semiconductor devices described in the above embodiments canbe applied to the display portion 9603. For example, a plurality of thinfilm transistors described in the above embodiments can be arranged asswitching elements in pixels.

FIG. 14B illustrates a digital photo frame 9700. For example, in adigital photo frame 9700, a display portion 9703 is incorporated in ahousing 9701. The display portion 9703 can display a variety of images.For example, the display portion 9703 can display image data taken witha digital camera or the like and function like a normal photo frame.

Any of the semiconductor devices described in the above embodiments canbe applied to the display portion 9703. For example, a plurality of thinfilm transistors described in the above embodiments can be arranged asswitching elements in pixels.

Note that the digital photo frame 9700 is provided with an operationportion, an external connection terminal (a USB terminal, a terminalconnectable to a variety of cables such as a USB cable), a storagemedium insertion portion, and the like. Although these components may beprovided on the same surface as the display portion, it is preferable toprovide them on the side surface or the back surface for designaesthetics. For example, a storage medium storing image data taken witha digital camera is inserted into the storage medium insertion portionof the digital photo frame and the data is loaded, whereby the image canbe displayed on the display portion 9703.

The digital photo frame 9700 may be configured to transmit and receivedata wirelessly. Through wireless communication, desired image data canbe loaded to be displayed.

FIG. 15 illustrates a portable game console including two housings, ahousing 9881 and a housing 9891 which are jointed with a joint portion9893 so that the portable game console can be opened or folded. Adisplay portion 9882 and a display portion 9883 are incorporated in thehousing 9881 and the housing 9891, respectively.

Any of the semiconductor devices described in the above embodiments canbe applied to the display portion 9883. For example, a plurality of thinfilm transistors described in the above embodiments can be arranged asswitching elements in pixels.

In addition, the portable game console illustrated in FIG. 15 isprovided with a speaker portion 9884, a storage medium insertion portion9886, an LED lamp 9890, input means (operation keys 9885, a connectionterminal 9887, a sensor 9888 (having a function of measuring force,displacement, position, speed, acceleration, angular velocity, rotationnumber, distance, light, liquid, magnetism, temperature, chemicalsubstance, sound, time, hardness, electric field, current, voltage,electric power, radial ray, flow rate, humidity, gradient, vibration,smell, or infrared ray), and a microphone 9889), and the like. Needlessto say, the structure of the portable game console is not limited to theabove and another structure which is provided with at least the thinfilm transistor disclosed in this specification can be employed. Theportable game console may include an additional accessory asappropriate. The portable game console illustrated in FIG. 15 has afunction of reading a program or data stored in a storage medium todisplay it on the display portion, and a function of sharing data withanother portable game console by wireless communication. Note that afunction of the portable game console illustrated in FIG. 15 is notlimited to those described above, and the portable game console can havea variety of functions.

FIG. 17 illustrates an example in which the light-emitting device whichis an example of the semiconductor device formed according to any of theabove embodiments is used as an indoor lighting device 3001. Since alight-emitting device described in this specification can have a largearea, the light-emitting device can be used as a lighting device havinga large emission area. In addition, any of the light-emitting devicesdescribed in the above embodiments can also be used as a desk lamp 3002.Note that the lighting equipment includes in its category, a ceilinglight, a wall light, a vehicle interior light, an emergency exit light,and the like.

As described above, the semiconductor device described in any ofEmbodiments 1 to 9 can be applied to a display panel of a variety ofelectronic devices described above and highly reliable electronicappliances can be provided.

Embodiment 15

The semiconductor device disclosed in this specification can be appliedto an electronic paper. An electronic paper can be used for electronicdevices for displaying information in all fields. For example, anelectronic paper can be applied to an electronic book reader (an e-bookreader), a poster, an advertisement in a vehicle such as a train, or adisplay of a variety of cards such as a credit card. FIG. 16 illustratesexamples of the electronic devices.

FIG. 16 illustrates an electronic book reader 2700. For example, anelectronic book reader 2700 includes two housings, a housing 2701 and ahousing 2703. The housing 2701 and the housing 2703 are combined with ahinge 2711 so that the electronic book reader 2700 can be opened andclosed along the hinge 2711. With such a structure, the electronic bookreader 2700 can be handled like a paper book.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703, respectively. The display portion2705 and the display portion 2707 may display one image or differentimages. In the case where the display portion 2705 and the displayportion 2707 display different images, for example, a display portion onthe right (the display portion 2705 in FIG. 16) can display text and adisplay portion on the left (the display portion 2707 in FIG. 16) candisplay an image.

FIG. 16 illustrates an example in which the housing 2701 is providedwith an operation portion and the like. For example, the housing 2701 isprovided with a power supply switch 2721, operation keys 2723, a speaker2725, and the like. Pages can be turned with the operation keys 2723.Note that a keyboard, a pointing device, and the like may be provided onthe same surface as the display portion of the housing. Moreover, anexternal connection terminal (an earphone terminal, a USB terminal, aterminal connectable to a variety of cables such as an AC adapter or aUSB cable), a storage medium insertion portion, and the like may beprovided on the back surface or the side surface of the housing.Moreover, the electronic book reader 2700 may have a function of anelectronic dictionary.

The electronic book reader 2700 may be configured to wirelessly transmitand receive data. Through wireless communication, desired book data orthe like can be purchased and downloaded from an electronic book server.

This embodiment can be implemented in appropriate combination withanother embodiment.

This application is based on Japanese Patent Application serial no.2009-249876 filed with Japan Patent Office on Oct. 30, 2009, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A semiconductor device comprising: a gate electrodelayer over a substrate; a gate insulating layer over the gate electrodelayer; an oxide semiconductor layer over the gate insulating layer; asource electrode layer and a drain electrode layer, each of the sourceelectrode layer and the drain electrode layer being in contact with theoxide semiconductor layer; an insulating layer over the source electrodelayer and the drain electrode layer, the insulating layer comprisingoxygen and silicon; and a first region between the oxide semiconductorlayer and the insulating layer, wherein the first region comprisesoxygen, silicon and at least one of metal elements included in the oxidesemiconductor layer.
 3. A semiconductor device according to claim 2,wherein the first region is in oxygen-excess state.
 4. A semiconductordevice according to claim 2, wherein the oxide semiconductor layercomprises indium, gallium and zinc.
 5. The semiconductor deviceaccording to claim 2, wherein the insulating layer has defects, andwherein the insulating layer is configured to bind and stabilize animpurity diffused from the oxide semiconductor layer.
 6. A semiconductordevice according to claim 2, wherein a thickness of the first region is0.1 nm to 30 nm.
 7. A semiconductor device according to claim 2, furthercomprising a protective insulating layer covering the insulating layer,wherein the gate insulating layer comprises a first layer over the gateelectrode layer and a second layer over the first layer, and wherein theprotective insulating layer is in contact with the first layer of thegate insulating layer.
 8. The semiconductor device according to claim 7,wherein each of the protective insulating layer and the first layer ofthe gate insulating layer contains nitrogen.
 9. A semiconductor devicecomprising: a gate electrode layer over a substrate; a gate insulatinglayer over the gate electrode layer; an oxide semiconductor layer overthe gate insulating layer; a source electrode layer and a drainelectrode layer, each of the source electrode layer and the drainelectrode layer being in contact with the oxide semiconductor layer; aninsulating layer over the source electrode layer and the drain electrodelayer, the insulating layer comprising oxygen and silicon; and a firstregion between the oxide semiconductor layer and the insulating layer,wherein the oxide semiconductor layer comprises indium, gallium andzinc, and wherein the first region comprises indium, gallium, zinc, andsilicon.
 10. A semiconductor device according to claim 9, wherein thefirst region is in oxygen-excess state.
 11. The semiconductor deviceaccording to claim 9, wherein the insulating layer has defects, andwherein the insulating layer is configured to bind and stabilize animpurity diffused from the oxide semiconductor layer.
 12. Asemiconductor device according to claim 9, wherein a thickness of thefirst region is 0.1 nm to 30 nm.
 13. A semiconductor device according toclaim 9, further comprising a protective insulating layer covering theinsulating layer, wherein the gate insulating layer comprises a firstlayer over the gate electrode layer and a second layer over the firstlayer, and wherein the protective insulating layer is in contact withthe first layer of the gate insulating layer.
 14. The semiconductordevice according to claim 13, wherein each of the protective insulatinglayer and the first layer of the gate insulating layer containsnitrogen.